diff --git a/noir-projects/aztec-nr/aztec/src/messaging.nr b/noir-projects/aztec-nr/aztec/src/messaging.nr
index db6e1809ac63..6e3943677096 100644
--- a/noir-projects/aztec-nr/aztec/src/messaging.nr
+++ b/noir-projects/aztec-nr/aztec/src/messaging.nr
@@ -15,7 +15,7 @@ pub fn process_l1_to_l2_message(
     content: Field,
     secret: Field
 ) -> Field {
-    let msg = L1ToL2Message::new(
+    let mut msg = L1ToL2Message::new(
         portal_contract_address,
         chain_id,
         storage_contract_address,
@@ -34,5 +34,12 @@ pub fn process_l1_to_l2_message(
     let root = compute_merkle_root(message_hash, leaf_index, sibling_path);
     assert(root == l1_to_l2_root, "Message not in state");
 
+    // Note: Had to add this line to make it work (wasted an hour debugging this). L1ToL2Message noir struct is
+    // an abomination and it would not have ever got to that state if people followed the logical rule of 
+    // "deserialize(serialize(object)) == object" always being true (there are a few params which are not
+    // serialzied). Will refactor that in a separate PR.
+    // TODO(#5420)
+    msg.tree_index = leaf_index;
+
     msg.compute_nullifier()
 }
diff --git a/noir-projects/aztec-nr/aztec/src/messaging/l1_to_l2_message.nr b/noir-projects/aztec-nr/aztec/src/messaging/l1_to_l2_message.nr
index 102989a456ab..6fd9a65e60c5 100644
--- a/noir-projects/aztec-nr/aztec/src/messaging/l1_to_l2_message.nr
+++ b/noir-projects/aztec-nr/aztec/src/messaging/l1_to_l2_message.nr
@@ -39,8 +39,8 @@ impl L1ToL2Message {
 
     pub fn deserialize(
         fields: [Field; L1_TO_L2_MESSAGE_LENGTH],
-        secret: Field,
-        tree_index: Field
+        secret: Field, // TODO(#5420): refactor this so that this param does not have to be passed separately
+        tree_index: Field // TODO(#5420): refactor this so that this param does not have to be passed separately
     ) -> L1ToL2Message {
         L1ToL2Message {
             sender: EthAddress::from_field(fields[0]),