diff --git a/optuna_integration/mlflow.py b/optuna_integration/mlflow.py
index 02963e43..e4691b80 100644
--- a/optuna_integration/mlflow.py
+++ b/optuna_integration/mlflow.py
@@ -150,7 +150,7 @@ def __call__(self, study: optuna.study.Study, trial: optuna.trial.FrozenTrial) -
             with mlflow.start_run(
                 run_id=trial.system_attrs.get(RUN_ID_ATTRIBUTE_KEY),
                 experiment_id=self._mlflow_kwargs.get("experiment_id"),
-                run_name=self._mlflow_kwargs.get("run_name") or str(trial.number),
+                run_name=self._mlflow_kwargs.get("run_name", str(trial.number)),
                 nested=self._mlflow_kwargs.get("nested") or False,
                 tags=self._mlflow_kwargs.get("tags"),
             ):
@@ -211,8 +211,9 @@ def wrapper(trial: optuna.trial.Trial) -> float | Sequence[float]:
                     study = trial.study
                     self._initialize_experiment(study)
                     nested = self._mlflow_kwargs.get("nested")
+                    run_name = self._mlflow_kwargs.get("run_name", str(trial.number))
 
-                    with mlflow.start_run(run_name=str(trial.number), nested=nested) as run:
+                    with mlflow.start_run(run_name=run_name, nested=nested) as run:
                         trial.storage.set_trial_system_attr(
                             trial._trial_id, RUN_ID_ATTRIBUTE_KEY, run.info.run_id
                         )
diff --git a/tests/test_mlflow.py b/tests/test_mlflow.py
index 6e80a98d..c9992409 100644
--- a/tests/test_mlflow.py
+++ b/tests/test_mlflow.py
@@ -188,7 +188,9 @@ def test_metric_name_multiobjective(
 def test_run_name(tmpdir: py.path.local, run_name: str | None, expected: str) -> None:
     tracking_uri = f"file:{tmpdir}"
 
-    mlflow_kwargs = {"run_name": run_name}
+    mlflow_kwargs = {}
+    if run_name is not None:
+        mlflow_kwargs = {"run_name": run_name}
     with warnings.catch_warnings():
         warnings.simplefilter("ignore", optuna.exceptions.ExperimentalWarning)
         mlflc = MLflowCallback(tracking_uri=tracking_uri, mlflow_kwargs=mlflow_kwargs)