diff --git a/mushroom_rl/environments/gym_env.py b/mushroom_rl/environments/gym_env.py
index 325c2d2b..a1c9834d 100644
--- a/mushroom_rl/environments/gym_env.py
+++ b/mushroom_rl/environments/gym_env.py
@@ -53,7 +53,7 @@ def __init__(self, name, horizon=None, gamma=0.99, wrappers=None, wrappers_args=
 
         if wrappers is not None:
             if wrappers_args is None:
-                wrappers_args = [dict()] * len(wrappers)
+                wrappers_args = [list()] * len(wrappers)
             for wrapper, args in zip(wrappers, wrappers_args):
                 if isinstance(wrapper, tuple):
                     self.env = wrapper[0](self.env, *args, **wrapper[1])