diff --git a/python/cocoindex/flow.py b/python/cocoindex/flow.py
index db53004f..d1378954 100644
--- a/python/cocoindex/flow.py
+++ b/python/cocoindex/flow.py
@@ -657,7 +657,7 @@ async def _update_flow(name: str, fl: Flow) -> tuple[str, _engine.IndexUpdateInf
 
 def _get_data_slice_annotation_type(data_slice_type: Type[DataSlice[T]]) -> Type[T] | None:
     type_args = get_args(data_slice_type)
-    if data_slice_type is DataSlice:
+    if data_slice_type is inspect.Parameter.empty or data_slice_type is DataSlice:
         return None
     if get_origin(data_slice_type) != DataSlice or len(type_args) != 1:
         raise ValueError(f"Expect a DataSlice[T] type, but got {data_slice_type}")