diff --git a/docs/misc/changelog.rst b/docs/misc/changelog.rst
index ed9c1dd..29f832e 100644
--- a/docs/misc/changelog.rst
+++ b/docs/misc/changelog.rst
@@ -18,6 +18,7 @@ New Features:
 
 Bug Fixes:
 ^^^^^^^^^^
+- Fixed MaskablePPO ignoring stats_window_size argument
 
 Deprecations:
 ^^^^^^^^^^^^^
diff --git a/sb3_contrib/ppo_mask/ppo_mask.py b/sb3_contrib/ppo_mask/ppo_mask.py
index ca23a22..ffd831c 100644
--- a/sb3_contrib/ppo_mask/ppo_mask.py
+++ b/sb3_contrib/ppo_mask/ppo_mask.py
@@ -222,8 +222,8 @@ class MaskablePPO(OnPolicyAlgorithm):
         self.start_time = time.time_ns()
         if self.ep_info_buffer is None or reset_num_timesteps:
             # Initialize buffers if they don't exist, or reinitialize if resetting counters
-            self.ep_info_buffer = deque(maxlen=100)
-            self.ep_success_buffer = deque(maxlen=100)
+            self.ep_info_buffer = deque(maxlen=self._stats_window_size)
+            self.ep_success_buffer = deque(maxlen=self._stats_window_size)
 
         if reset_num_timesteps:
             self.num_timesteps = 0
diff --git a/tests/test_run.py b/tests/test_run.py
index 9ce4c71..9da6525 100644
--- a/tests/test_run.py
+++ b/tests/test_run.py
@@ -161,12 +161,13 @@ def test_advantage_normalization(normalize_advantage):
     model.learn(64)
 
 
-@pytest.mark.parametrize("algo", [TRPO, QRDQN])
+@pytest.mark.parametrize("algo", [TRPO, QRDQN, MaskablePPO])
 @pytest.mark.parametrize("stats_window_size", [1, 42])
 def test_ep_buffers_stats_window_size(algo, stats_window_size):
     """Set stats_window_size for logging to non-default value and check if
     ep_info_buffer and ep_success_buffer are initialized to the correct length"""
-    model = algo("MlpPolicy", "CartPole-v1", stats_window_size=stats_window_size)
+    env = InvalidActionEnvDiscrete() if algo == MaskablePPO else "CartPole-v1"
+    model = algo("MlpPolicy", env, stats_window_size=stats_window_size)
     model.learn(total_timesteps=10)
     assert model.ep_info_buffer.maxlen == stats_window_size
     assert model.ep_success_buffer.maxlen == stats_window_size