diff --git a/optuna_dashboard/streamlit/_streamlit_helper.py b/optuna_dashboard/streamlit/_streamlit_helper.py
index 14f746fd..252c0901 100644
--- a/optuna_dashboard/streamlit/_streamlit_helper.py
+++ b/optuna_dashboard/streamlit/_streamlit_helper.py
@@ -83,7 +83,9 @@ def _render_widgets(
 
 
 def render_user_attr_form_widgets(
-    study: optuna.Study, trial: FrozenTrial, on_success_callback: Optional[Callable[[], None]]
+    study: optuna.Study,
+    trial: FrozenTrial,
+    on_success_callback: Optional[Callable[[], None]] = None,
 ) -> None:
     """Render user input widgets to UI with streamlit.
 
@@ -123,7 +125,9 @@ def render_user_attr_form_widgets(
 
 
 def render_objective_form_widgets(
-    study: optuna.Study, trial: FrozenTrial, on_success_callback: Optional[Callable[[], None]]
+    study: optuna.Study,
+    trial: FrozenTrial,
+    on_success_callback: Optional[Callable[[], None]] = None,
 ) -> None:
     """Render user input widgets to UI with streamlit.