diff --git a/hathor/transaction/aux_pow.py b/hathor/transaction/aux_pow.py
index 795d9a9f7..0a18ee2aa 100644
--- a/hathor/transaction/aux_pow.py
+++ b/hathor/transaction/aux_pow.py
@@ -58,8 +58,9 @@ def verify(self, _base_block_hash: bytes) -> None:
             raise AuxPowNoMagicError('cannot find MAGIC_NUMBER')
         if magic_index < len(self.coinbase_head) - len(MAGIC_NUMBER):
             raise AuxPowUnexpectedMagicError('unexpected MAGIC_NUMBER')
-        if len(self.merkle_path) > MAX_MERKLE_PATH_LENGTH:
-            raise AuxPowLongMerklePathError('`merkle_path` too long')
+        merkle_path_length = len(self.merkle_path)
+        if merkle_path_length > MAX_MERKLE_PATH_LENGTH:
+            raise AuxPowLongMerklePathError(f'merkle_path too long: {merkle_path_length} > {MAX_MERKLE_PATH_LENGTH}')
 
     def __bytes__(self) -> bytes:
         """ Convert to byte representation.