diff --git a/docs/misc/changelog.rst b/docs/misc/changelog.rst
index 65c1487..c097a82 100644
--- a/docs/misc/changelog.rst
+++ b/docs/misc/changelog.rst
@@ -45,6 +45,7 @@ Others:
 - Fixed ``stable_baselines3/common/type_aliases.py`` type hint
 - Fixed ``stable_baselines3/common/torch_layers.py`` type hint
 - Fixed ``stable_baselines3/common/env_util.py`` type hint
+- Fixed ``stable_baselines3/common/preprocessing.py`` type hints
 - Exposed modules in ``__init__.py`` with the ``__all__`` attribute (@ZikangXiong)
 - Upgraded GitHub CI/setup-python to v4 and checkout to v3
 
diff --git a/setup.cfg b/setup.cfg
index 045638c..f56f005 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -44,7 +44,6 @@ exclude = (?x)(
     | stable_baselines3/common/off_policy_algorithm.py$
     | stable_baselines3/common/on_policy_algorithm.py$
     | stable_baselines3/common/policies.py$
-    | stable_baselines3/common/preprocessing.py$
     | stable_baselines3/common/save_util.py$
     | stable_baselines3/common/sb2_compat/rmsprop_tf_like.py$
     | stable_baselines3/common/utils.py$
diff --git a/stable_baselines3/common/preprocessing.py b/stable_baselines3/common/preprocessing.py
index a406a7d..76dab70 100644
--- a/stable_baselines3/common/preprocessing.py
+++ b/stable_baselines3/common/preprocessing.py
@@ -122,6 +122,7 @@ def preprocess_obs(
 
     elif isinstance(observation_space, spaces.Dict):
         # Do not modify by reference the original observation
+        assert isinstance(obs, Dict), f"Expected dict, got {type(obs)}"
         preprocessed_obs = {}
         for key, _obs in obs.items():
             preprocessed_obs[key] = preprocess_obs(_obs, observation_space[key], normalize_images=normalize_images)
@@ -155,7 +156,7 @@ def get_obs_shape(
         else:
             return (int(observation_space.n),)
     elif isinstance(observation_space, spaces.Dict):
-        return {key: get_obs_shape(subspace) for (key, subspace) in observation_space.spaces.items()}
+        return {key: get_obs_shape(subspace) for (key, subspace) in observation_space.spaces.items()}  # type: ignore[misc]
 
     else:
         raise NotImplementedError(f"{observation_space} observation space is not supported")