diff --git a/flytekit/core/type_engine.py b/flytekit/core/type_engine.py
index ed52ee8bf5..edb7bea65f 100644
--- a/flytekit/core/type_engine.py
+++ b/flytekit/core/type_engine.py
@@ -505,6 +505,9 @@ def _fix_val_int(self, t: typing.Type, val: typing.Any) -> typing.Any:
 
         if get_origin(t) is typing.Union and type(None) in get_args(t):
             # Handle optional type. e.g. Optional[int], Optional[dataclass]
+            # Marshmallow doesn't support union type, so the type here is always an optional type.
+            # https://github.com/marshmallow-code/marshmallow/issues/1191#issuecomment-480831796
+            # Note: Typing.Union[None, int] is also an optional type, but Marshmallow does not support it.
             return self._fix_val_int(get_args(t)[0], val)
 
         if dataclasses.is_dataclass(t):
diff --git a/tests/flytekit/unit/core/test_type_engine.py b/tests/flytekit/unit/core/test_type_engine.py
index e37b30834b..0fe2513908 100644
--- a/tests/flytekit/unit/core/test_type_engine.py
+++ b/tests/flytekit/unit/core/test_type_engine.py
@@ -149,7 +149,7 @@ def test_list_of_dataclass_getting_python_value():
     @dataclass()
     class Bar(object):
         v: typing.Union[int, None]
-        w: typing.Union[None, str]
+        w: typing.Optional[str]
         x: float
         y: str
         z: typing.Dict[str, bool]