diff --git a/src/core/headers_store.cairo b/src/core/headers_store.cairo
index 03b3dce..389a50c 100644
--- a/src/core/headers_store.cairo
+++ b/src/core/headers_store.cairo
@@ -138,6 +138,9 @@ trait IHeadersStore<TContractState> {
     // @param mmr_id The id of the MMR to clone
     // @param last_pos last_pos of the given MMR
     // @param new_mmr_id The id of the new MMR
+    // @dev Notice that to prevent overwriting existing MMRs, a check if the MMR with the new id
+    // already exists is performed. Because of that MMR root = 0 is reserved for non-existing MMRs,
+    // so 0 cannot be used as a valid mmr root anywhere.
     fn create_branch_from(
         ref self: TContractState, mmr_id: MmrId, last_pos: MmrSize, new_mmr_id: MmrId
     );
diff --git a/src/core/tests/test_headers_store.cairo b/src/core/tests/test_headers_store.cairo
index d248c2d..608e583 100644
--- a/src/core/tests/test_headers_store.cairo
+++ b/src/core/tests/test_headers_store.cairo
@@ -256,8 +256,7 @@ fn test_create_branch_from_message() {
 
     // Root equal to 0 is not allowed.
     assert(
-        dispatcher.create_branch_from_message(0, 1, 0, mmr_id_2).is_err(),
-        'Root = 0 should fail'
+        dispatcher.create_branch_from_message(0, 1, 0, mmr_id_2).is_err(), 'Root = 0 should fail'
     );
 
     // Creating MMR with the same ID should fail.