diff --git a/zou/app/blueprints/crud/base.py b/zou/app/blueprints/crud/base.py
index fe615c1b30..887cb502d8 100644
--- a/zou/app/blueprints/crud/base.py
+++ b/zou/app/blueprints/crud/base.py
@@ -79,7 +79,11 @@ def build_filters(self, options):
                 ) and isinstance(
                     field_key.property, orm.properties.RelationshipProperty
                 )
-                value_is_list = len(value) > 0 and value[0] == "["
+                value_is_list = (
+                    hasattr(value, "__len__")
+                    and len(value) > 0
+                    and value[0] == "["
+                )
 
                 if key == "name" and field_key is not None:
                     name_filter.append(value)
diff --git a/zou/app/utils/query.py b/zou/app/utils/query.py
index 062a48ef41..f78de28775 100644
--- a/zou/app/utils/query.py
+++ b/zou/app/utils/query.py
@@ -40,7 +40,11 @@ def apply_criterions_to_db_query(model, db_query, criterions):
             ) and isinstance(
                 field_key.property, orm.properties.RelationshipProperty
             )
-            value_is_list = len(value) > 0 and value[0] == "["
+            value_is_list = (
+                hasattr(value, "__len__")
+                and len(value) > 0
+                and value[0] == "["
+            )
 
             if key == "name" and field_key is not None:
                 name_filter.append(value)