diff --git a/fuse/data/ops/ops_common.py b/fuse/data/ops/ops_common.py
index fe55a186..9b70bfe0 100644
--- a/fuse/data/ops/ops_common.py
+++ b/fuse/data/ops/ops_common.py
@@ -718,7 +718,7 @@ def __call__(
         key_in: str,
         find_any_val: List[Any],
         replace_with_val: Any,
-        key_out: str = None,
+        key_out: Optional[str] = None,
     ) -> NDict:
         """
         Args: