diff --git a/optuna_dashboard/preferential/_study.py b/optuna_dashboard/preferential/_study.py
index 57ab8e9f..8895d604 100644
--- a/optuna_dashboard/preferential/_study.py
+++ b/optuna_dashboard/preferential/_study.py
@@ -276,7 +276,9 @@ class PreferentialStudy:
         to generate a new trial if this method returns :obj:`True`, and to wait for human
         evaluation if this method returns :obj:`False`.
         """
-        return len(get_active_trials(self._study._study_id, self._study._storage)) < self.n_generate
+        return (
+            len(get_active_trials(self._study._study_id, self._study._storage)) < self.n_generate
+        )
 
 
 def get_active_trials(study_id: int, storage: optuna.storages.BaseStorage) -> list[FrozenTrial]:
diff --git a/python_tests/test_api.py b/python_tests/test_api.py
index ae50e29a..fc67e12e 100644
--- a/python_tests/test_api.py
+++ b/python_tests/test_api.py
@@ -108,6 +108,8 @@ class APITestCase(TestCase):
             study.mark_comparison_ready(trial)
         study.report_preference(study.trials[0], study.trials[1])
 
+        assert len(study.best_trials) == 1
+
         app = create_app(storage)
         study_id = study._study._study_id
         status, _, body = send_request(
@@ -119,9 +121,8 @@ class APITestCase(TestCase):
         self.assertEqual(status, 200)
 
         best_trials = json.loads(body)["best_trials"]
-        assert len(best_trials) == 2
+        assert len(best_trials) == 1
         assert best_trials[0]["number"] == 0
-        assert best_trials[1]["number"] == 2
 
     def test_report_preference(self) -> None:
         storage = optuna.storages.InMemoryStorage()