diff --git a/optunahub/benchmarks/_base_problem.py b/optunahub/benchmarks/_base_problem.py
index 92599a4..cc8c48e 100644
--- a/optunahub/benchmarks/_base_problem.py
+++ b/optunahub/benchmarks/_base_problem.py
@@ -69,7 +69,7 @@ def directions(self) -> list[optuna.study.StudyDirection]:
         """Return the optimization directions.
 
         Returns:
-            List of `optuna.study.direction <https://optuna.readthedocs.io/en/stable/reference/generated/optuna.study.StudyDirection.html>`__.
+            List of `optuna.study.StudyDirection <https://optuna.readthedocs.io/en/stable/reference/generated/optuna.study.StudyDirection.html>`__.
 
         Example:
             ::