diff --git a/optuna_dashboard/_serializer.py b/optuna_dashboard/_serializer.py
index 4a804af8..a037290e 100644
--- a/optuna_dashboard/_serializer.py
+++ b/optuna_dashboard/_serializer.py
@@ -108,7 +108,9 @@ def serialize_study_summary(summary: StudySummary) -> dict[str, Any]:
         "study_name": summary.study_name,
         "directions": [d.name.lower() for d in summary.directions],
         "user_attrs": serialize_attrs(summary.user_attrs),
-        "is_preferential": summary.system_attrs.get(_SYSTEM_ATTR_PREFERENTIAL_STUDY, False),
+        "is_preferential": getattr(summary, "_system_attrs", {}).get(
+            _SYSTEM_ATTR_PREFERENTIAL_STUDY, False
+        ),
     }
 
     if summary.datetime_start is not None: