diff --git a/optuna_integration/mlflow/mlflow.py b/optuna_integration/mlflow/mlflow.py
index 431f7fee..197e8264 100644
--- a/optuna_integration/mlflow/mlflow.py
+++ b/optuna_integration/mlflow/mlflow.py
@@ -153,7 +153,7 @@ def wrapper(trial: optuna.trial.Trial) -> float | Sequence[float]:
                 with self._lock:
                     study = trial.study
                     self._initialize_experiment(study)
-                    nested = self._mlflow_kwargs.get("nested")
+                    nested = bool(self._mlflow_kwargs.get("nested"))
                     run_name = self._mlflow_kwargs.get("run_name", str(trial.number))
 
                     with mlflow.start_run(run_name=run_name, nested=nested) as run:
diff --git a/tests/mlflow/test_mlflow.py b/tests/mlflow/test_mlflow.py
index c9992409..0082fd19 100644
--- a/tests/mlflow/test_mlflow.py
+++ b/tests/mlflow/test_mlflow.py
@@ -127,7 +127,8 @@ def test_use_existing_experiment_by_id(tmpdir: py.path.local) -> None:
     assert experiment.experiment_id == experiment_id
     assert experiment.name == "foo"
 
-    runs = mlfl_client.search_runs(experiment_id)
+    # TODO(y0z): Remove type ignore once the MLFlow typing is fixed.
+    runs = mlfl_client.search_runs(experiment_id)  # type: ignore
     assert len(runs) == 10
 
 
@@ -235,7 +236,7 @@ def test_tag_truncation(tmpdir: py.path.local) -> None:
     first_run_dict = first_run.to_dictionary()
 
     my_user_attr = first_run_dict["data"]["tags"]["my_user_attr"]
-    assert len(my_user_attr) <= 5000
+    assert len(my_user_attr) <= 8000
 
 
 def test_nest_trials(tmpdir: py.path.local) -> None: