diff --git a/optuna_dashboard/_cached_extra_study_property.py b/optuna_dashboard/_cached_extra_study_property.py
index 919853f4..24e22372 100644
--- a/optuna_dashboard/_cached_extra_study_property.py
+++ b/optuna_dashboard/_cached_extra_study_property.py
@@ -3,7 +3,6 @@ from __future__ import annotations
 import copy
 import numbers
 import threading
-from typing import Any
 from typing import List
 from typing import Optional
 from typing import Set
diff --git a/optuna_dashboard/_serializer.py b/optuna_dashboard/_serializer.py
index dbcdc991..7030abec 100644
--- a/optuna_dashboard/_serializer.py
+++ b/optuna_dashboard/_serializer.py
@@ -7,6 +7,7 @@ from typing import Any
 from typing import TYPE_CHECKING
 from typing import Union
 
+import numpy as np
 from optuna.distributions import BaseDistribution
 from optuna.distributions import CategoricalDistribution
 from optuna.study import StudySummary