diff --git a/src/network_management/network_manager.rs b/src/network_management/network_manager.rs
index 7314bd1..764e08d 100644
--- a/src/network_management/network_manager.rs
+++ b/src/network_management/network_manager.rs
@@ -158,7 +158,7 @@ impl NetworkManager {
                     self.get_control_function_address_by_name(destination.get_name()),
                     priority,
                 )
-                .unwrap_or(CanId::default());
+                .unwrap_or_default();
 
                 if message_id.raw() != CanId::default().raw() {
                     self.enqueue_can_message(
diff --git a/src/object_pool/mod.rs b/src/object_pool/mod.rs
index c12c414..5b0a3b0 100644
--- a/src/object_pool/mod.rs
+++ b/src/object_pool/mod.rs
@@ -2,7 +2,7 @@ pub mod colour;
 pub mod reader;
 pub mod writer;
 
-mod object;
+pub mod object;
 mod object_attributes;
 mod object_id;
 mod object_pool;
@@ -12,6 +12,9 @@ mod vt_version;
 use crate::network_management::name::NAME;
 
 pub use colour::Colour;
+pub use object_attributes::ObjectRef;
+pub use object_id::NullableObjectId;
+pub use object_id::ObjectId;
 pub use object_pool::ObjectPool;
 pub use object_type::ObjectType;
 
@@ -19,4 +22,7 @@ pub use object_type::ObjectType;
 pub enum ParseError {
     DataEmpty,
     UnknownObjectType,
+    UnexpectedNullObjectId,
+    BooleanOutOfRange,
+    UnsupportedVtVersion,
 }
diff --git a/src/object_pool/object.rs b/src/object_pool/object.rs
index 39bdb66..eb4acb0 100644
--- a/src/object_pool/object.rs
+++ b/src/object_pool/object.rs
@@ -11,6 +11,8 @@ use crate::object_pool::object_attributes::{
 use crate::object_pool::object_id::ObjectId;
 use crate::object_pool::{Colour, ObjectType};
 
+use super::object_id::NullableObjectId;
+
 #[derive(Debug)]
 pub enum Object {
     WorkingSet(WorkingSet),
@@ -26,6 +28,7 @@ pub enum Object {
     InputList(InputList),
     OutputString(OutputString),
     OutputNumber(OutputNumber),
+    OutputList(OutputList),
     OutputLine(OutputLine),
     OutputRectangle(OutputRectangle),
     OutputEllipse(OutputEllipse),
@@ -50,7 +53,6 @@ pub enum Object {
     WindowMask(WindowMask),
     KeyGroup(KeyGroup),
     GraphicsContext(GraphicsContext),
-    OutputList(OutputList),
     ExtendedInputAttributes(ExtendedInputAttributes),
     ColourMap(ColourMap),
     ObjectLabelReferenceList(ObjectLabelReferenceList),
@@ -80,6 +82,7 @@ impl Object {
             Object::InputList(o) => o.id,
             Object::OutputString(o) => o.id,
             Object::OutputNumber(o) => o.id,
+            Object::OutputList(o) => o.id,
             Object::OutputLine(o) => o.id,
             Object::OutputRectangle(o) => o.id,
             Object::OutputEllipse(o) => o.id,
@@ -104,7 +107,6 @@ impl Object {
             Object::WindowMask(o) => o.id,
             Object::KeyGroup(o) => o.id,
             Object::GraphicsContext(o) => o.id,
-            Object::OutputList(o) => o.id,
             Object::ExtendedInputAttributes(o) => o.id,
             Object::ColourMap(o) => o.id,
             Object::ObjectLabelReferenceList(o) => o.id,
@@ -134,6 +136,7 @@ impl Object {
             Object::InputList(_) => ObjectType::InputList,
             Object::OutputString(_) => ObjectType::OutputString,
             Object::OutputNumber(_) => ObjectType::OutputNumber,
+            Object::OutputList(_) => ObjectType::OutputList,
             Object::OutputLine(_) => ObjectType::OutputLine,
             Object::OutputRectangle(_) => ObjectType::OutputRectangle,
             Object::OutputEllipse(_) => ObjectType::OutputEllipse,
@@ -160,7 +163,6 @@ impl Object {
             Object::WindowMask(_) => ObjectType::WindowMask,
             Object::KeyGroup(_) => ObjectType::KeyGroup,
             Object::GraphicsContext(_) => ObjectType::GraphicsContext,
-            Object::OutputList(_) => ObjectType::OutputList,
             Object::ExtendedInputAttributes(_) => ObjectType::ExtendedInputAttributes,
             Object::ColourMap(_) => ObjectType::ColourMap,
             Object::ObjectLabelReferenceList(_) => ObjectType::ObjectLabelReferenceList,
@@ -174,6 +176,171 @@ impl Object {
             Object::ScaledGraphic(_) => ObjectType::ScaledGraphic,
         }
     }
+
+    ///
+    /// Returns a list of object IDs referenced by this object in the order they are listed
+    /// as attributes in the object specification.
+    ///
+    /// # Examples
+    /// We can use this method recursively to process objects in a "depth-first" manner
+    /// as required by the ISO 11783-6 (ch. 4.6.1.3) standard:
+    /// ```
+    /// use ag_iso_stack::object_pool::ObjectId;
+    /// use ag_iso_stack::object_pool::ObjectPool;
+    ///
+    /// fn process_object(object_pool: &ObjectPool, object_id: ObjectId) {
+    ///     let object = object_pool.object_by_id(object_id).unwrap();
+    ///     for referenced_object_id in object.referenced_objects() {
+    ///         process_object(object_pool, referenced_object_id);
+    ///     }
+    ///     // Process the object here
+    /// }
+    ///
+    /// let working_set = object_pool.working_set_object().unwrap();
+    /// process_object(&object_pool, working_set.id);
+    ///
+    pub fn referenced_objects(&self) -> Vec<ObjectId> {
+        let mut refs: Vec<ObjectId> = vec![];
+
+        fn extend_object_refs(refs: &mut Vec<ObjectId>, object_refs: &[ObjectRef]) {
+            refs.extend(object_refs.iter().map(|x| x.id));
+        }
+
+        fn push_nullable_id(refs: &mut Vec<ObjectId>, nullable_id: &NullableObjectId) {
+            if let Some(id) = nullable_id.0 {
+                refs.push(id);
+            }
+        }
+
+        fn extend_nullable_ids(refs: &mut Vec<ObjectId>, nullable_ids: &[NullableObjectId]) {
+            refs.extend(nullable_ids.iter().filter_map(|x| x.0));
+        }
+
+        match self {
+            Object::WorkingSet(o) => {
+                refs.push(o.active_mask);
+                extend_object_refs(&mut refs, &o.object_refs);
+            }
+            Object::DataMask(o) => {
+                push_nullable_id(&mut refs, &o.soft_key_mask);
+                extend_object_refs(&mut refs, &o.object_refs);
+            }
+            Object::AlarmMask(o) => {
+                push_nullable_id(&mut refs, &o.soft_key_mask);
+                extend_object_refs(&mut refs, &o.object_refs);
+            }
+            Object::Container(o) => extend_object_refs(&mut refs, &o.object_refs),
+            Object::SoftKeyMask(o) => refs.extend(&o.objects),
+            Object::Key(o) => extend_object_refs(&mut refs, &o.object_refs),
+            Object::Button(o) => extend_object_refs(&mut refs, &o.object_refs),
+            Object::InputBoolean(o) => {
+                refs.push(o.foreground_colour);
+                push_nullable_id(&mut refs, &o.variable_reference);
+            }
+            Object::InputString(o) => {
+                refs.push(o.font_attributes);
+                push_nullable_id(&mut refs, &o.input_attributes);
+                push_nullable_id(&mut refs, &o.variable_reference);
+            }
+            Object::InputNumber(o) => {
+                refs.push(o.font_attributes);
+                push_nullable_id(&mut refs, &o.variable_reference);
+            }
+            Object::InputList(o) => {
+                push_nullable_id(&mut refs, &o.variable_reference);
+                extend_nullable_ids(&mut refs, &o.list_items);
+            }
+            Object::OutputString(o) => {
+                refs.push(o.font_attributes);
+                push_nullable_id(&mut refs, &o.variable_reference);
+            }
+            Object::OutputNumber(o) => {
+                refs.push(o.font_attributes);
+                push_nullable_id(&mut refs, &o.variable_reference);
+            }
+            Object::OutputList(o) => {
+                push_nullable_id(&mut refs, &o.variable_reference);
+                extend_nullable_ids(&mut refs, &o.list_items);
+            }
+            Object::OutputLine(o) => refs.push(o.line_attributes),
+            Object::OutputRectangle(o) => {
+                refs.push(o.line_attributes);
+                push_nullable_id(&mut refs, &o.fill_attributes);
+            }
+            Object::OutputEllipse(o) => {
+                refs.push(o.line_attributes);
+                push_nullable_id(&mut refs, &o.fill_attributes);
+            }
+            Object::OutputPolygon(o) => {
+                refs.push(o.line_attributes);
+                push_nullable_id(&mut refs, &o.fill_attributes);
+            }
+            Object::OutputMeter(o) => push_nullable_id(&mut refs, &o.variable_reference),
+            Object::OutputLinearBarGraph(o) => {
+                push_nullable_id(&mut refs, &o.variable_reference);
+                push_nullable_id(&mut refs, &o.target_value_variable_reference);
+            }
+            Object::OutputArchedBarGraph(o) => {
+                push_nullable_id(&mut refs, &o.variable_reference);
+                push_nullable_id(&mut refs, &o.target_value_variable_reference);
+            }
+            Object::PictureGraphic(_)
+            | Object::NumberVariable(_)
+            | Object::StringVariable(_)
+            | Object::FontAttributes(_)
+            | Object::LineAttributes(_) => (), // No references
+            Object::FillAttributes(o) => push_nullable_id(&mut refs, &o.fill_pattern),
+            Object::InputAttributes(_) => (), // No references
+            Object::ObjectPointer(o) => push_nullable_id(&mut refs, &o.value),
+            Object::Macro(_)
+            | Object::AuxiliaryFunctionType1(_)
+            | Object::AuxiliaryInputType1(_)
+            | Object::AuxiliaryFunctionType2(_)
+            | Object::AuxiliaryInputType2(_) => (), // No references
+            Object::AuxiliaryControlDesignatorType2(o) => refs.push(o.auxiliary_object_id),
+            Object::WindowMask(o) => {
+                push_nullable_id(&mut refs, &o.window_title);
+                push_nullable_id(&mut refs, &o.window_icon);
+                extend_nullable_ids(&mut refs, &o.objects);
+                extend_object_refs(&mut refs, &o.object_refs);
+            }
+            Object::KeyGroup(o) => {
+                refs.push(o.name);
+                push_nullable_id(&mut refs, &o.key_group_icon);
+                refs.extend(&o.objects);
+            }
+            Object::GraphicsContext(o) => {
+                push_nullable_id(&mut refs, &o.font_attributes_object);
+                push_nullable_id(&mut refs, &o.line_attributes_object);
+                push_nullable_id(&mut refs, &o.fill_attributes_object);
+            }
+            Object::ExtendedInputAttributes(_) | Object::ColourMap(_) => (), // No references
+            Object::ObjectLabelReferenceList(o) => {
+                for label in o.object_labels.as_slice() {
+                    refs.push(label.id);
+                    push_nullable_id(&mut refs, &label.string_variable_reference);
+                    push_nullable_id(&mut refs, &label.graphic_representation);
+                }
+            }
+            Object::ExternalObjectDefinition(o) => extend_nullable_ids(&mut refs, &o.objects),
+            Object::ExternalReferenceName(_) => (), // No references
+            Object::ExternalObjectPointer(o) => {
+                push_nullable_id(&mut refs, &o.default_object_id);
+                push_nullable_id(&mut refs, &o.external_reference_name_id);
+                push_nullable_id(&mut refs, &o.external_object_id);
+            }
+            Object::Animation(o) => extend_object_refs(&mut refs, &o.object_refs),
+
+            Object::ColourPalette(_) | Object::GraphicData(_) => (), // No references
+            Object::WorkingSetSpecialControls(o) => {
+                push_nullable_id(&mut refs, &o.id_of_colour_map);
+                push_nullable_id(&mut refs, &o.id_of_colour_palette);
+            }
+            Object::ScaledGraphic(o) => push_nullable_id(&mut refs, &o.value),
+        };
+
+        refs
+    }
 }
 
 #[derive(Debug, PartialEq)]
@@ -191,7 +358,7 @@ pub struct WorkingSet {
 pub struct DataMask {
     pub id: ObjectId,
     pub background_colour: u8,
-    pub soft_key_mask: ObjectId,
+    pub soft_key_mask: NullableObjectId,
     pub object_refs: Vec<ObjectRef>,
     pub macro_refs: Vec<MacroRef>,
 }
@@ -200,7 +367,7 @@ pub struct DataMask {
 pub struct AlarmMask {
     pub id: ObjectId,
     pub background_colour: u8,
-    pub soft_key_mask: ObjectId,
+    pub soft_key_mask: NullableObjectId,
     pub priority: u8,
     pub acoustic_signal: u8,
     pub object_refs: Vec<ObjectRef>,
@@ -253,7 +420,7 @@ pub struct InputBoolean {
     pub background_colour: u8,
     pub width: u16,
     pub foreground_colour: ObjectId,
-    pub variable_reference: ObjectId,
+    pub variable_reference: NullableObjectId,
     pub value: bool,
     pub enabled: bool,
     pub macro_refs: Vec<MacroRef>,
@@ -266,9 +433,9 @@ pub struct InputString {
     pub height: u16,
     pub background_colour: u8,
     pub font_attributes: ObjectId,
-    pub input_attributes: ObjectId,
+    pub input_attributes: NullableObjectId,
     pub options: InputStringOptions,
-    pub variable_reference: ObjectId,
+    pub variable_reference: NullableObjectId,
     pub justification: Alignment,
     pub value: String,
     pub enabled: bool,
@@ -283,7 +450,7 @@ pub struct InputNumber {
     pub background_colour: u8,
     pub font_attributes: ObjectId,
     pub options: NumberOptions,
-    pub variable_reference: ObjectId,
+    pub variable_reference: NullableObjectId,
     pub value: u32,
     pub min_value: u32,
     pub max_value: u32,
@@ -301,10 +468,10 @@ pub struct InputList {
     pub id: ObjectId,
     pub width: u16,
     pub height: u16,
-    pub variable_reference: ObjectId,
+    pub variable_reference: NullableObjectId,
     pub value: u8,
     pub options: InputListOptions,
-    pub list_items: Vec<ObjectId>,
+    pub list_items: Vec<NullableObjectId>,
     pub macro_refs: Vec<MacroRef>,
 }
 
@@ -316,7 +483,7 @@ pub struct OutputString {
     pub background_colour: u8,
     pub font_attributes: ObjectId,
     pub options: OutputStringOptions,
-    pub variable_reference: ObjectId,
+    pub variable_reference: NullableObjectId,
     pub justification: Alignment,
     pub value: String,
     pub macro_refs: Vec<MacroRef>,
@@ -330,7 +497,7 @@ pub struct OutputNumber {
     pub background_colour: u8,
     pub font_attributes: ObjectId,
     pub options: NumberOptions,
-    pub variable_reference: ObjectId,
+    pub variable_reference: NullableObjectId,
     pub value: u32,
     pub offset: i32,
     pub scale: f32,
@@ -345,9 +512,9 @@ pub struct OutputList {
     pub id: ObjectId,
     pub width: u16,
     pub height: u16,
-    pub variable_reference: ObjectId,
+    pub variable_reference: NullableObjectId,
     pub value: u8,
-    pub list_items: Vec<ObjectId>,
+    pub list_items: Vec<NullableObjectId>,
     pub macro_refs: Vec<MacroRef>,
 }
 
@@ -368,7 +535,7 @@ pub struct OutputRectangle {
     pub width: u16,
     pub height: u16,
     pub line_suppression: u8,
-    pub fill_attributes: ObjectId,
+    pub fill_attributes: NullableObjectId,
     pub macro_refs: Vec<MacroRef>,
 }
 
@@ -381,7 +548,7 @@ pub struct OutputEllipse {
     pub ellipse_type: u8,
     pub start_angle: u8,
     pub end_angle: u8,
-    pub fill_attributes: ObjectId,
+    pub fill_attributes: NullableObjectId,
     pub macro_refs: Vec<MacroRef>,
 }
 
@@ -391,7 +558,7 @@ pub struct OutputPolygon {
     pub width: u16,
     pub height: u16,
     pub line_attributes: ObjectId,
-    pub fill_attributes: ObjectId,
+    pub fill_attributes: NullableObjectId,
     pub polygon_type: u8,
     pub points: Vec<Point<u16>>,
     pub macro_refs: Vec<MacroRef>,
@@ -410,7 +577,7 @@ pub struct OutputMeter {
     pub end_angle: u8,
     pub min_value: u16,
     pub max_value: u16,
-    pub variable_reference: ObjectId,
+    pub variable_reference: NullableObjectId,
     pub value: u16,
     pub macro_refs: Vec<MacroRef>,
 }
@@ -426,9 +593,9 @@ pub struct OutputLinearBarGraph {
     pub nr_of_ticks: u8,
     pub min_value: u16,
     pub max_value: u16,
-    pub variable_reference: ObjectId,
+    pub variable_reference: NullableObjectId,
     pub value: u16,
-    pub target_value_variable_reference: ObjectId,
+    pub target_value_variable_reference: NullableObjectId,
     pub target_value: u16,
     pub macro_refs: Vec<MacroRef>,
 }
@@ -446,9 +613,9 @@ pub struct OutputArchedBarGraph {
     pub bar_graph_width: u16,
     pub min_value: u16,
     pub max_value: u16,
-    pub variable_reference: ObjectId,
+    pub variable_reference: NullableObjectId,
     pub value: u16,
-    pub target_value_variable_reference: ObjectId,
+    pub target_value_variable_reference: NullableObjectId,
     pub target_value: u16,
     pub macro_refs: Vec<MacroRef>,
 }
@@ -502,7 +669,7 @@ pub struct FillAttributes {
     pub id: ObjectId,
     pub fill_type: u8,
     pub fill_colour: u8,
-    pub fill_pattern: ObjectId,
+    pub fill_pattern: NullableObjectId,
     pub macro_refs: Vec<MacroRef>,
 }
 
@@ -561,7 +728,7 @@ pub struct ExtendedInputAttributes {
 #[derive(Debug)]
 pub struct ObjectPointer {
     pub id: ObjectId,
-    pub value: ObjectId,
+    pub value: NullableObjectId,
 }
 
 #[derive(Debug)]
@@ -630,9 +797,9 @@ pub struct GraphicsContext {
     pub graphics_cursor_y: i16,
     pub foreground_colour: u8,
     pub background_colour: u8,
-    pub font_attributes_object: ObjectId,
-    pub line_attributes_object: ObjectId,
-    pub fill_attributes_object: ObjectId,
+    pub font_attributes_object: NullableObjectId,
+    pub line_attributes_object: NullableObjectId,
+    pub fill_attributes_object: NullableObjectId,
     pub format: ColorFormat,
     pub options: GraphicsContextOptions,
     pub transparency_colour: u8,
@@ -646,9 +813,9 @@ pub struct WindowMask {
     pub background_colour: u8,
     pub options: WindowMaskOptions,
     pub name: ObjectId,
-    pub window_title: ObjectId,
-    pub window_icon: ObjectId,
-    pub objects: Vec<ObjectId>,
+    pub window_title: NullableObjectId,
+    pub window_icon: NullableObjectId,
+    pub objects: Vec<NullableObjectId>,
     pub object_refs: Vec<ObjectRef>,
     pub macro_refs: Vec<MacroRef>,
 }
@@ -658,7 +825,7 @@ pub struct KeyGroup {
     pub id: ObjectId,
     pub options: KeyGroupOptions,
     pub name: ObjectId,
-    pub key_group_icon: ObjectId,
+    pub key_group_icon: NullableObjectId,
     pub objects: Vec<ObjectId>,
     pub macro_refs: Vec<MacroRef>,
 }
@@ -674,7 +841,7 @@ pub struct ExternalObjectDefinition {
     pub id: ObjectId,
     pub options: ExternalObjectDefinitionOptions,
     pub name: NAME,
-    pub objects: Vec<ObjectId>,
+    pub objects: Vec<NullableObjectId>,
 }
 
 #[derive(Debug)]
@@ -687,9 +854,9 @@ pub struct ExternalReferenceName {
 #[derive(Debug)]
 pub struct ExternalObjectPointer {
     pub id: ObjectId,
-    pub default_object_id: ObjectId,
-    pub external_reference_name_id: ObjectId,
-    pub external_object_id: ObjectId,
+    pub default_object_id: NullableObjectId,
+    pub external_reference_name_id: NullableObjectId,
+    pub external_object_id: NullableObjectId,
 }
 
 #[derive(Debug)]
@@ -729,14 +896,14 @@ pub struct ScaledGraphic {
     pub height: u16,
     pub scale_type: u8,
     pub options: ScaledGraphicOptions,
-    pub value: u16,
+    pub value: NullableObjectId,
     pub macro_refs: Vec<MacroRef>,
 }
 
 #[derive(Debug)]
 pub struct WorkingSetSpecialControls {
     pub id: ObjectId,
-    pub id_of_colour_map: ObjectId,
-    pub id_of_colour_palette: ObjectId,
+    pub id_of_colour_map: NullableObjectId,
+    pub id_of_colour_palette: NullableObjectId,
     pub language_pairs: Vec<(String, String)>,
 }
diff --git a/src/object_pool/object_attributes.rs b/src/object_pool/object_attributes.rs
index 3be6550..ce42ed0 100644
--- a/src/object_pool/object_attributes.rs
+++ b/src/object_pool/object_attributes.rs
@@ -5,6 +5,8 @@ use bitvec::vec::BitVec;
 use bitvec::view::BitView;
 use strum_macros::FromRepr;
 
+use super::object_id::NullableObjectId;
+
 #[derive(FromRepr, Debug, PartialEq, Clone, Copy)]
 #[repr(u8)]
 pub enum WindowType {
@@ -168,9 +170,9 @@ impl core::ops::Add<Point<i16>> for Point<u16> {
 #[derive(Debug)]
 pub struct ObjectLabel {
     pub id: ObjectId,
-    pub string_variable_reference: ObjectId,
+    pub string_variable_reference: NullableObjectId,
     pub font_type: u8,
-    pub graphic_representation: ObjectId,
+    pub graphic_representation: NullableObjectId,
 }
 
 #[derive(Debug, PartialEq, Clone, Copy)]
diff --git a/src/object_pool/object_id.rs b/src/object_pool/object_id.rs
index 747649e..fd42461 100644
--- a/src/object_pool/object_id.rs
+++ b/src/object_pool/object_id.rs
@@ -10,14 +10,15 @@ impl ObjectId {
 
     pub fn new(id: u16) -> Result<Self, ParseError> {
         if id == Self::NULL.id {
-            Err(ParseError::UnknownObjectType)
+            Err(ParseError::UnexpectedNullObjectId)
         } else {
             Ok(ObjectId { id })
         }
     }
 }
 
-pub struct NullableObjectId(Option<ObjectId>);
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct NullableObjectId(pub Option<ObjectId>);
 
 impl NullableObjectId {
     pub const NULL: NullableObjectId = NullableObjectId(None);
@@ -74,6 +75,12 @@ impl TryFrom<u16> for ObjectId {
     }
 }
 
+impl From<NullableObjectId> for Option<ObjectId> {
+    fn from(id: NullableObjectId) -> Self {
+        return id.0;
+    }
+}
+
 impl From<ObjectId> for u16 {
     fn from(val: ObjectId) -> Self {
         val.id
diff --git a/src/object_pool/object_pool.rs b/src/object_pool/object_pool.rs
index 1f6bee7..59edab3 100644
--- a/src/object_pool/object_pool.rs
+++ b/src/object_pool/object_pool.rs
@@ -79,15 +79,23 @@ impl ObjectPool {
     where
         I: IntoIterator<Item = u8>,
     {
-        let mut data = data.into_iter();
-
         let mut op = Self::new();
+        op.extend_with_iop(data);
+        op
+    }
+
+    pub fn extend_with_iop<I>(&mut self, data: I)
+    where
+        I: IntoIterator<Item = u8>,
+    {
+        let mut data = data.into_iter();
 
         while let Ok(o) = Object::read(&mut data) {
-            op.objects.push(o);
+            // By the standard, if there already is an object with the same ID, the new object
+            // replaces the old one
+            self.objects.retain(|x| x.id() != o.id());
+            self.objects.push(o);
         }
-
-        op
     }
 
     pub fn as_iop(&self) -> Vec<u8> {
diff --git a/src/object_pool/reader.rs b/src/object_pool/reader.rs
index ec56f50..df58023 100644
--- a/src/object_pool/reader.rs
+++ b/src/object_pool/reader.rs
@@ -1,3 +1,4 @@
+use super::object_id::NullableObjectId;
 use super::*;
 use crate::object_pool::colour::Colour;
 use crate::object_pool::object::*;
@@ -70,7 +71,7 @@ impl Object {
 
     /* READ COMMON TYPES */
 
-    fn read_objects(
+    fn read_object_ids(
         data: &mut dyn Iterator<Item = u8>,
         nr_of_objects: usize,
     ) -> Result<Vec<ObjectId>, ParseError> {
@@ -81,6 +82,17 @@ impl Object {
         Ok(objs)
     }
 
+    fn read_nullable_object_ids(
+        data: &mut dyn Iterator<Item = u8>,
+        nr_of_objects: usize,
+    ) -> Result<Vec<NullableObjectId>, ParseError> {
+        let mut objs = Vec::new();
+        for _ in 0..nr_of_objects {
+            objs.push(Self::read_u16(data)?.into());
+        }
+        Ok(objs)
+    }
+
     fn read_object_refs(
         data: &mut dyn Iterator<Item = u8>,
         nr_of_objects: usize,
@@ -156,9 +168,9 @@ impl Object {
         for _ in 0..nr_of_objects {
             objs.push(ObjectLabel {
                 id: Self::read_u16(data)?.try_into()?,
-                string_variable_reference: Self::read_u16(data)?.try_into()?,
+                string_variable_reference: Self::read_u16(data)?.into(),
                 font_type: Self::read_u8(data)?,
-                graphic_representation: Self::read_u16(data)?.try_into()?,
+                graphic_representation: Self::read_u16(data)?.into(),
             })
         }
         Ok(objs)
@@ -215,7 +227,7 @@ impl Object {
                 if d == 0 || d == 1 {
                     Ok(d == 1)
                 } else {
-                    Err(ParseError::UnknownObjectType)
+                    Err(ParseError::BooleanOutOfRange)
                 }
             }
             None => Err(ParseError::DataEmpty),
@@ -300,6 +312,7 @@ impl Object {
     }
     fn read_string(len: usize, data: &mut dyn Iterator<Item = u8>) -> Result<String, ParseError> {
         let mut s = String::new();
+
         for _ in 0..len {
             if let Some(c) = data.next() {
                 s.push(c as char);
@@ -363,7 +376,7 @@ impl Object {
         let mut o = DataMask {
             id,
             background_colour: Self::read_u8(data)?,
-            soft_key_mask: Self::read_u16(data)?.try_into()?,
+            soft_key_mask: Self::read_u16(data)?.into(),
             object_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
@@ -383,7 +396,7 @@ impl Object {
         let mut o = AlarmMask {
             id,
             background_colour: Self::read_u8(data)?,
-            soft_key_mask: Self::read_u16(data)?.try_into()?,
+            soft_key_mask: Self::read_u16(data)?.into(),
             priority: Self::read_u8(data)?,
             acoustic_signal: Self::read_u8(data)?,
             object_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
@@ -431,7 +444,7 @@ impl Object {
         };
 
         o.objects
-            .extend(Self::read_objects(data, o.objects.capacity())?);
+            .extend(Self::read_object_ids(data, o.objects.capacity())?);
         o.macro_refs
             .extend(Self::read_macro_refs(data, o.macro_refs.capacity())?);
 
@@ -485,7 +498,7 @@ impl Object {
             background_colour: Self::read_u8(data)?,
             width: Self::read_u16(data)?,
             foreground_colour: Self::read_u16(data)?.try_into()?,
-            variable_reference: Self::read_u16(data)?.try_into()?,
+            variable_reference: Self::read_u16(data)?.into(),
             value: Self::read_bool(data)?,
             enabled: Self::read_bool(data)?,
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
@@ -507,9 +520,9 @@ impl Object {
             height: Self::read_u16(data)?,
             background_colour: Self::read_u8(data)?,
             font_attributes: Self::read_u16(data)?.try_into()?,
-            input_attributes: Self::read_u16(data)?.try_into()?,
+            input_attributes: Self::read_u16(data)?.into(),
             options: Self::read_u8(data)?.into(),
-            variable_reference: Self::read_u16(data)?.try_into()?,
+            variable_reference: Self::read_u16(data)?.into(),
             justification: Self::read_u8(data)?.into(),
             value: Self::read_string(Self::read_u8(data)?.into(), data)?,
             enabled: Self::read_bool(data)?,
@@ -533,7 +546,7 @@ impl Object {
             background_colour: Self::read_u8(data)?,
             font_attributes: Self::read_u16(data)?.try_into()?,
             options: Self::read_u8(data)?.into(),
-            variable_reference: Self::read_u16(data)?.try_into()?,
+            variable_reference: Self::read_u16(data)?.into(),
             value: Self::read_u32(data)?,
             min_value: Self::read_u32(data)?,
             max_value: Self::read_u32(data)?,
@@ -560,15 +573,17 @@ impl Object {
             id,
             width: Self::read_u16(data)?,
             height: Self::read_u16(data)?,
-            variable_reference: Self::read_u16(data)?.try_into()?,
+            variable_reference: Self::read_u16(data)?.into(),
             value: Self::read_u8(data)?,
             list_items: Vec::with_capacity(Self::read_u8(data)?.into()),
             options: Self::read_u8(data)?.into(),
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
 
-        o.list_items
-            .extend(Self::read_objects(data, o.list_items.capacity())?);
+        o.list_items.extend(Self::read_nullable_object_ids(
+            data,
+            o.list_items.capacity(),
+        )?);
         o.macro_refs
             .extend(Self::read_macro_refs(data, o.macro_refs.capacity())?);
 
@@ -586,7 +601,7 @@ impl Object {
             background_colour: Self::read_u8(data)?,
             font_attributes: Self::read_u16(data)?.try_into()?,
             options: Self::read_u8(data)?.into(),
-            variable_reference: Self::read_u16(data)?.try_into()?,
+            variable_reference: Self::read_u16(data)?.into(),
             justification: Self::read_u8(data)?.into(),
             value: Self::read_string(Self::read_u16(data)?.into(), data)?,
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
@@ -609,7 +624,7 @@ impl Object {
             background_colour: Self::read_u8(data)?,
             font_attributes: Self::read_u16(data)?.try_into()?,
             options: Self::read_u8(data)?.into(),
-            variable_reference: Self::read_u16(data)?.try_into()?,
+            variable_reference: Self::read_u16(data)?.into(),
             value: Self::read_u32(data)?,
             offset: Self::read_i32(data)?,
             scale: Self::read_f32(data)?,
@@ -654,7 +669,7 @@ impl Object {
             width: Self::read_u16(data)?,
             height: Self::read_u16(data)?,
             line_suppression: Self::read_u8(data)?,
-            fill_attributes: Self::read_u16(data)?.try_into()?,
+            fill_attributes: Self::read_u16(data)?.into(),
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
 
@@ -676,7 +691,7 @@ impl Object {
             ellipse_type: Self::read_u8(data)?,
             start_angle: Self::read_u8(data)?,
             end_angle: Self::read_u8(data)?,
-            fill_attributes: Self::read_u16(data)?.try_into()?,
+            fill_attributes: Self::read_u16(data)?.into(),
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
 
@@ -695,7 +710,7 @@ impl Object {
             width: Self::read_u16(data)?,
             height: Self::read_u16(data)?,
             line_attributes: Self::read_u16(data)?.try_into()?,
-            fill_attributes: Self::read_u16(data)?.try_into()?,
+            fill_attributes: Self::read_u16(data)?.into(),
             polygon_type: Self::read_u8(data)?,
             points: Vec::with_capacity(Self::read_u8(data)?.into()),
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
@@ -725,7 +740,7 @@ impl Object {
             end_angle: Self::read_u8(data)?,
             min_value: Self::read_u16(data)?,
             max_value: Self::read_u16(data)?,
-            variable_reference: Self::read_u16(data)?.try_into()?,
+            variable_reference: Self::read_u16(data)?.into(),
             value: Self::read_u16(data)?,
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
@@ -750,9 +765,9 @@ impl Object {
             nr_of_ticks: Self::read_u8(data)?,
             min_value: Self::read_u16(data)?,
             max_value: Self::read_u16(data)?,
-            variable_reference: Self::read_u16(data)?.try_into()?,
+            variable_reference: Self::read_u16(data)?.into(),
             value: Self::read_u16(data)?,
-            target_value_variable_reference: Self::read_u16(data)?.try_into()?,
+            target_value_variable_reference: Self::read_u16(data)?.into(),
             target_value: Self::read_u16(data)?,
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
@@ -779,9 +794,9 @@ impl Object {
             bar_graph_width: Self::read_u16(data)?,
             min_value: Self::read_u16(data)?,
             max_value: Self::read_u16(data)?,
-            variable_reference: Self::read_u16(data)?.try_into()?,
+            variable_reference: Self::read_u16(data)?.into(),
             value: Self::read_u16(data)?,
-            target_value_variable_reference: Self::read_u16(data)?.try_into()?,
+            target_value_variable_reference: Self::read_u16(data)?.into(),
             target_value: Self::read_u16(data)?,
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
@@ -884,7 +899,7 @@ impl Object {
             id,
             fill_type: Self::read_u8(data)?,
             fill_colour: Self::read_u8(data)?,
-            fill_pattern: Self::read_u16(data)?.try_into()?,
+            fill_pattern: Self::read_u16(data)?.into(),
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
 
@@ -917,7 +932,7 @@ impl Object {
     ) -> Result<Self, ParseError> {
         let o = ObjectPointer {
             id,
-            value: Self::read_u16(data)?.try_into()?,
+            value: Self::read_u16(data)?.into(),
         };
 
         Ok(Object::ObjectPointer(o))
@@ -1028,15 +1043,15 @@ impl Object {
             background_colour: Self::read_u8(data)?,
             options: Self::read_u8(data)?.into(),
             name: Self::read_u16(data)?.try_into()?,
-            window_title: Self::read_u16(data)?.try_into()?,
-            window_icon: Self::read_u16(data)?.try_into()?,
+            window_title: Self::read_u16(data)?.into(),
+            window_icon: Self::read_u16(data)?.into(),
             objects: Vec::with_capacity(Self::read_u8(data)?.into()),
             object_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
 
         o.objects
-            .extend(Self::read_objects(data, o.objects.capacity())?);
+            .extend(Self::read_nullable_object_ids(data, o.objects.capacity())?);
         o.object_refs
             .extend(Self::read_object_refs(data, o.object_refs.capacity())?);
         o.macro_refs
@@ -1053,13 +1068,13 @@ impl Object {
             id,
             options: Self::read_u8(data)?.into(),
             name: Self::read_u16(data)?.try_into()?,
-            key_group_icon: Self::read_u16(data)?.try_into()?,
+            key_group_icon: Self::read_u16(data)?.into(),
             objects: Vec::with_capacity(Self::read_u8(data)?.into()),
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
 
         o.objects
-            .extend(Self::read_objects(data, o.objects.capacity())?);
+            .extend(Self::read_object_ids(data, o.objects.capacity())?);
         o.macro_refs
             .extend(Self::read_macro_refs(data, o.macro_refs.capacity())?);
 
@@ -1083,9 +1098,9 @@ impl Object {
             graphics_cursor_y: Self::read_i16(data)?,
             foreground_colour: Self::read_u8(data)?,
             background_colour: Self::read_u8(data)?,
-            font_attributes_object: Self::read_u16(data)?.try_into()?,
-            line_attributes_object: Self::read_u16(data)?.try_into()?,
-            fill_attributes_object: Self::read_u16(data)?.try_into()?,
+            font_attributes_object: Self::read_u16(data)?.into(),
+            line_attributes_object: Self::read_u16(data)?.into(),
+            fill_attributes_object: Self::read_u16(data)?.into(),
             format: Self::read_u8(data)?.into(),
             options: Self::read_u8(data)?.into(),
             transparency_colour: Self::read_u8(data)?,
@@ -1101,14 +1116,16 @@ impl Object {
             id,
             width: Self::read_u16(data)?,
             height: Self::read_u16(data)?,
-            variable_reference: Self::read_u16(data)?.try_into()?,
+            variable_reference: Self::read_u16(data)?.into(),
             value: Self::read_u8(data)?,
             list_items: Vec::with_capacity(Self::read_u8(data)?.into()),
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
 
-        o.list_items
-            .extend(Self::read_objects(data, o.list_items.capacity())?);
+        o.list_items.extend(Self::read_nullable_object_ids(
+            data,
+            o.list_items.capacity(),
+        )?);
         o.macro_refs
             .extend(Self::read_macro_refs(data, o.macro_refs.capacity())?);
 
@@ -1170,7 +1187,7 @@ impl Object {
         };
 
         o.objects
-            .extend(Self::read_objects(data, o.objects.capacity())?);
+            .extend(Self::read_nullable_object_ids(data, o.objects.capacity())?);
 
         Ok(Object::ExternalObjectDefinition(o))
     }
@@ -1194,9 +1211,9 @@ impl Object {
     ) -> Result<Self, ParseError> {
         let o = ExternalObjectPointer {
             id,
-            default_object_id: Self::read_u16(data)?.try_into()?,
-            external_reference_name_id: Self::read_u16(data)?.try_into()?,
-            external_object_id: Self::read_u16(data)?.try_into()?,
+            default_object_id: Self::read_u16(data)?.into(),
+            external_reference_name_id: Self::read_u16(data)?.into(),
+            external_object_id: Self::read_u16(data)?.into(),
         };
 
         Ok(Object::ExternalObjectPointer(o))
@@ -1266,8 +1283,8 @@ impl Object {
     ) -> Result<Self, ParseError> {
         let mut o = WorkingSetSpecialControls {
             id,
-            id_of_colour_map: Self::read_u16(data)?.try_into()?,
-            id_of_colour_palette: Self::read_u16(data)?.try_into()?,
+            id_of_colour_map: Self::read_u16(data)?.into(),
+            id_of_colour_palette: Self::read_u16(data)?.into(),
             language_pairs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
 
@@ -1289,7 +1306,7 @@ impl Object {
             height: Self::read_u16(data)?,
             scale_type: Self::read_u8(data)?,
             options: Self::read_u8(data)?.into(),
-            value: Self::read_u16(data)?,
+            value: Self::read_u16(data)?.into(),
             macro_refs: Vec::with_capacity(Self::read_u8(data)?.into()),
         };
 
diff --git a/src/object_pool/vt_version.rs b/src/object_pool/vt_version.rs
index 16410ab..da1f63a 100644
--- a/src/object_pool/vt_version.rs
+++ b/src/object_pool/vt_version.rs
@@ -1,5 +1,5 @@
 use crate::object_pool::ParseError;
-use crate::object_pool::ParseError::UnknownObjectType;
+use crate::object_pool::ParseError::UnsupportedVtVersion;
 
 #[derive(Debug, Default)]
 pub enum VtVersion {
@@ -39,7 +39,7 @@ impl TryFrom<u8> for VtVersion {
             4 => Ok(VtVersion::Version4),
             5 => Ok(VtVersion::Version5),
             6 => Ok(VtVersion::Version6),
-            _ => Err(UnknownObjectType),
+            _ => Err(UnsupportedVtVersion),
         }
     }
 }
diff --git a/src/object_pool/writer.rs b/src/object_pool/writer.rs
index c2edb9d..91a8edb 100644
--- a/src/object_pool/writer.rs
+++ b/src/object_pool/writer.rs
@@ -1,3 +1,4 @@
+use super::object_id::NullableObjectId;
 use super::*;
 use crate::object_pool::colour::Colour;
 use crate::object_pool::object::{
@@ -84,11 +85,18 @@ impl Object {
         data
     }
 
-    fn write_objects(data: &mut Vec<u8>, objects: &Vec<ObjectId>) {
+    fn write_object_ids(data: &mut Vec<u8>, objects: &Vec<ObjectId>) {
         for d in objects {
             Self::write_u16(data, *d);
         }
     }
+
+    fn write_nullable_object_ids(data: &mut Vec<u8>, objects: &Vec<NullableObjectId>) {
+        for d in objects {
+            Self::write_u16(data, *d);
+        }
+    }
+
     fn write_object_refs(data: &mut Vec<u8>, object_refs: &Vec<ObjectRef>) {
         for d in object_refs {
             Self::write_u16(data, d.id);
@@ -247,7 +255,7 @@ impl Object {
         Self::write_u8(data, o.objects.len() as u8);
         Self::write_u8(data, o.macro_refs.len() as u8);
 
-        Self::write_objects(data, &o.objects);
+        Self::write_object_ids(data, &o.objects);
         Self::write_macro_refs(data, &o.macro_refs);
     }
     fn write_key(data: &mut Vec<u8>, o: &Key) {
@@ -339,7 +347,7 @@ impl Object {
         Self::write_u8(data, o.options);
         Self::write_u8(data, o.macro_refs.len() as u8);
 
-        Self::write_objects(data, &o.list_items);
+        Self::write_nullable_object_ids(data, &o.list_items);
         Self::write_macro_refs(data, &o.macro_refs);
     }
     fn write_output_string(data: &mut Vec<u8>, o: &OutputString) {
@@ -628,7 +636,7 @@ impl Object {
         Self::write_u8(data, o.object_refs.len() as u8);
         Self::write_u8(data, o.macro_refs.len() as u8);
 
-        Self::write_objects(data, &o.objects);
+        Self::write_nullable_object_ids(data, &o.objects);
         Self::write_object_refs(data, &o.object_refs);
         Self::write_macro_refs(data, &o.macro_refs);
     }
@@ -641,7 +649,7 @@ impl Object {
         Self::write_u8(data, o.objects.len() as u8);
         Self::write_u8(data, o.macro_refs.len() as u8);
 
-        Self::write_objects(data, &o.objects);
+        Self::write_object_ids(data, &o.objects);
         Self::write_macro_refs(data, &o.macro_refs);
     }
     fn write_graphics_context(data: &mut Vec<u8>, o: &GraphicsContext) {
@@ -675,7 +683,7 @@ impl Object {
         Self::write_u8(data, o.list_items.len() as u8);
         Self::write_u8(data, o.macro_refs.len() as u8);
 
-        Self::write_objects(data, &o.list_items);
+        Self::write_nullable_object_ids(data, &o.list_items);
         Self::write_macro_refs(data, &o.macro_refs);
     }
     fn write_extended_input_attributes(data: &mut Vec<u8>, o: &ExtendedInputAttributes) {
@@ -705,7 +713,7 @@ impl Object {
         Self::write_name(data, o.name);
         Self::write_u8(data, o.objects.len() as u8);
 
-        Self::write_objects(data, &o.objects);
+        Self::write_nullable_object_ids(data, &o.objects);
     }
     fn write_external_reference_name(data: &mut Vec<u8>, o: &ExternalReferenceName) {
         Self::write_u16(data, o.id);