diff --git a/flytekit/core/promise.py b/flytekit/core/promise.py
index 2a3687c06c..12cd86d4af 100644
--- a/flytekit/core/promise.py
+++ b/flytekit/core/promise.py
@@ -619,7 +619,7 @@ def binding_data_from_python_std(
             f"Cannot pass output from task {t_value.task_name} that produces no outputs to a downstream task"
         )
 
-    elif expected_literal_type.union_type is not None:
+    elif t_value is not None and expected_literal_type.union_type is not None:
         for i in range(len(expected_literal_type.union_type.variants)):
             try:
                 lt_type = expected_literal_type.union_type.variants[i]
diff --git a/tests/flytekit/unit/core/test_promise.py b/tests/flytekit/unit/core/test_promise.py
index 9478cc33ba..b214764c50 100644
--- a/tests/flytekit/unit/core/test_promise.py
+++ b/tests/flytekit/unit/core/test_promise.py
@@ -145,3 +145,18 @@ def t1(a: typing.Union[float, typing.Dict[str, int]]):
             t1.interface.inputs,
             t1.python_interface.inputs,
         )
+
+
+def test_optional_task_kwargs():
+    from typing import Optional
+
+    from flytekit import Workflow
+
+    @task
+    def func(foo: Optional[int] = None):
+        pass
+
+    wf = Workflow(name="test")
+    wf.add_entity(func, foo=None)
+
+    wf()