diff --git a/optuna/visualization/_intermediate_values.py b/optuna/visualization/_intermediate_values.py
index 59e338afc4..89b0189591 100644
--- a/optuna/visualization/_intermediate_values.py
+++ b/optuna/visualization/_intermediate_values.py
@@ -132,7 +132,7 @@ def _get_intermediate_plot(info: _IntermediatePlotInfo) -> "go.Figure":
             mode="lines+markers",
             marker=default_marker
             if tinfo.feasible
-            else {**default_marker, "color": "#CCCCCC"},
+            else {**default_marker, "color": "#CCCCCC"},  # type: ignore[dict-item]
             name="Trial{}".format(tinfo.trial_number),
         )
         for tinfo in trial_infos
diff --git a/tests/visualization_tests/test_intermediate_plot.py b/tests/visualization_tests/test_intermediate_plot.py
index f1d1954f04..a2d19939ea 100644
--- a/tests/visualization_tests/test_intermediate_plot.py
+++ b/tests/visualization_tests/test_intermediate_plot.py
@@ -115,7 +115,7 @@ def constraints(trial: FrozenTrial) -> Sequence[float]:
                 ),
                 _TrialInfo(
                     trial_number=1, sorted_intermediate_values=[(1, 2.0), (0, 1.0)], feasible=False
-                ) 
+                ),
             ]
         ),
     ],