diff --git a/torchrl/envs/common.py b/torchrl/envs/common.py
index f5d102a6279..3b55fd227a7 100644
--- a/torchrl/envs/common.py
+++ b/torchrl/envs/common.py
@@ -561,7 +561,7 @@ def check_env_specs(self, *args, **kwargs):
 
     check_env_specs.__doc__ = check_env_specs_func.__doc__
 
-    def cardinality(self, tensordict: TensorDictBase|None=None) -> int:
+    def cardinality(self, tensordict: TensorDictBase | None = None) -> int:
         """The cardinality of the action space.
 
         By default, this is just a wrapper around :meth:`env.action_space.cardinality <~torchrl.data.TensorSpec.cardinality>`.