diff --git a/optuna_dashboard/_cached_extra_study_property.py b/optuna_dashboard/_cached_extra_study_property.py
index 0046336c..2ff86901 100644
--- a/optuna_dashboard/_cached_extra_study_property.py
+++ b/optuna_dashboard/_cached_extra_study_property.py
@@ -62,7 +62,7 @@ class _CachedExtraStudyProperty:
         return union
 
     @property
-    def union_user_attrs(self) -> Set[Tuple[str, Any]]:
+    def union_user_attrs(self) -> List[Tuple[str, Any]]:
         union = list(self._union_user_attrs)
         union.sort(key=lambda x: x[0])
         return union