diff --git a/src/error.rs b/src/error.rs
index 771b833..d4245da 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -1,7 +1,7 @@
 #[cfg(feature = "std")]
 use std::{error::Error, fmt::Display};
 
-use crate::{bonsai_database::DBError, String};
+use crate::{bonsai_database::DBError, BitVec, String};
 
 /// All errors that can be returned by BonsaiStorage.
 #[derive(Debug)]
@@ -22,7 +22,7 @@ where
     /// Error when decoding a node
     NodeDecodeError(parity_scale_codec::Error),
     /// Error when creating a storage proof.
-    CreateProof(String),
+    CreateProofKeyNotInTree { key: BitVec },
     /// Malformated trie key.
     KeyLength { expected: usize, got: usize },
 }
@@ -56,7 +56,9 @@ where
             BonsaiStorageError::Merge(e) => write!(f, "Merge error: {}", e),
             BonsaiStorageError::Database(e) => write!(f, "Database error: {}", e),
             BonsaiStorageError::NodeDecodeError(e) => write!(f, "Node decode error: {}", e),
-            BonsaiStorageError::CreateProof(e) => write!(f, "Proof creation error: {}", e),
+            BonsaiStorageError::CreateProofKeyNotInTree { key } => {
+                write!(f, "Key not in tree: {key:b}")
+            }
             BonsaiStorageError::KeyLength { expected, got } => {
                 write!(f, "Malformated key length: expected {expected}, got {got}")
             }
diff --git a/src/lib.rs b/src/lib.rs
index cf0a5a8..6662ae9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -128,7 +128,7 @@ pub mod id;
 
 pub use bonsai_database::{BonsaiDatabase, BonsaiPersistentDatabase, DBError, DatabaseKey};
 pub use error::BonsaiStorageError;
-pub use trie::proof::Membership;
+pub use trie::proof::{Membership, MultiProof, ProofNode};
 
 #[cfg(test)]
 mod tests;
@@ -153,7 +153,7 @@ impl<T: parity_scale_codec::Encode> EncodeExt for T {}
 use changes::ChangeBatch;
 use key_value_db::KeyValueDB;
 use starknet_types_core::{felt::Felt, hash::StarkHash};
-use trie::{proof::MultiProof, tree::bytes_to_bitvec, trees::MerkleTrees};
+use trie::{tree::bytes_to_bitvec, trees::MerkleTrees};
 
 /// Structure that contains the configuration for the BonsaiStorage.
 /// A default implementation is provided with coherent values.
diff --git a/src/trie/proof.rs b/src/trie/proof.rs
index b108937..71b1daa 100644
--- a/src/trie/proof.rs
+++ b/src/trie/proof.rs
@@ -4,7 +4,6 @@ use super::{
     tree::MerkleTree,
 };
 use crate::{
-    format,
     id::Id,
     key_value_db::KeyValueDB,
     trie::{
@@ -71,7 +70,7 @@ impl MultiProof {
         key_values.into_iter().map(move |(k, v)| {
             let k = k.as_ref();
 
-            if k.len() != tree_height as _ {
+            if k.len() != tree_height as usize {
                 return Membership::NonMember;
             }
 
@@ -187,11 +186,15 @@ impl<H: StarkHash + Send + Sync> MerkleTree<H> {
                     got: key.len(),
                 });
             }
+            log::debug!("go to = {key:b}");
             iter.traverse_to(&mut visitor, key)?;
+
+            log::debug!("iter = {iter:?}");
             // We should have found a leaf here.
-            iter.leaf_hash.ok_or_else(|| {
-                BonsaiStorageError::CreateProof(format!("Key {key:b} is not in the trie."))
-            })?;
+            iter.leaf_hash
+                .ok_or(BonsaiStorageError::CreateProofKeyNotInTree {
+                    key: key.to_bitvec(),
+                })?;
         }
 
         Ok(visitor.0)