diff --git a/optuna_dashboard/_importance.py b/optuna_dashboard/_importance.py
index dd67050d..aef90a63 100644
--- a/optuna_dashboard/_importance.py
+++ b/optuna_dashboard/_importance.py
@@ -61,7 +61,7 @@ def get_param_importance_from_trials_cache(
     cache_key = f"{study_id}:{objective_id}"
     with param_importance_cache_lock:
         cache_n_trial, cache_importance = param_importance_cache.get(
-            cache_key, [0, {"target_name": target_name, "param_importances": []}]
+            cache_key, (0, {"target_name": target_name, "param_importances": []})
         )
         if n_completed_trials == cache_n_trial:
             return cache_importance