diff --git a/docs/misc/changelog.rst b/docs/misc/changelog.rst
index cb3a17f..d8b6d6d 100644
--- a/docs/misc/changelog.rst
+++ b/docs/misc/changelog.rst
@@ -4,7 +4,7 @@ Changelog
 ==========
 
 
-Release 1.5.1a5 (WIP)
+Release 1.5.1a6 (WIP)
 ---------------------------
 
 Breaking Changes:
@@ -25,6 +25,7 @@ Bug Fixes:
 - Fixed final goal selection strategy that did not sample the final achieved goal (@qgallouedec)
 - Fixed a bug with special characters in the tensorboard log name (@quantitative-technologies)
 - Fixed a bug in ``DummyVecEnv``'s and ``SubprocVecEnv``'s seeding function. None value was unchecked (@ScheiklP)
+- Fixed a bug where ``EvalCallback`` would crash when trying to synchronize ``VecNormalize`` stats when observation normalization was disabled
 
 Deprecations:
 ^^^^^^^^^^^^^
diff --git a/stable_baselines3/common/vec_env/__init__.py b/stable_baselines3/common/vec_env/__init__.py
index 37ebc36..3880fbd 100644
--- a/stable_baselines3/common/vec_env/__init__.py
+++ b/stable_baselines3/common/vec_env/__init__.py
@@ -66,7 +66,9 @@ def sync_envs_normalization(env: "GymEnv", eval_env: "GymEnv") -> None:
     env_tmp, eval_env_tmp = env, eval_env
     while isinstance(env_tmp, VecEnvWrapper):
         if isinstance(env_tmp, VecNormalize):
-            eval_env_tmp.obs_rms = deepcopy(env_tmp.obs_rms)
+            # Only synchronize if observation normalization exists
+            if hasattr(env_tmp, "obs_rms"):
+                eval_env_tmp.obs_rms = deepcopy(env_tmp.obs_rms)
             eval_env_tmp.ret_rms = deepcopy(env_tmp.ret_rms)
         env_tmp = env_tmp.venv
         eval_env_tmp = eval_env_tmp.venv
diff --git a/stable_baselines3/version.txt b/stable_baselines3/version.txt
index bccb8c6..1e5deca 100644
--- a/stable_baselines3/version.txt
+++ b/stable_baselines3/version.txt
@@ -1 +1 @@
-1.5.1a5
+1.5.1a6
diff --git a/tests/test_vec_normalize.py b/tests/test_vec_normalize.py
index 86a0d84..a363e40 100644
--- a/tests/test_vec_normalize.py
+++ b/tests/test_vec_normalize.py
@@ -388,11 +388,11 @@ def test_offpolicy_normalization(model_class, online_sampling):
 
 @pytest.mark.parametrize("make_env", [make_env, make_dict_env])
 def test_sync_vec_normalize(make_env):
-    env = DummyVecEnv([make_env])
+    original_env = DummyVecEnv([make_env])
 
-    assert unwrap_vec_normalize(env) is None
+    assert unwrap_vec_normalize(original_env) is None
 
-    env = VecNormalize(env, norm_obs=True, norm_reward=True, clip_obs=100.0, clip_reward=100.0)
+    env = VecNormalize(original_env, norm_obs=True, norm_reward=True, clip_obs=100.0, clip_reward=100.0)
 
     assert isinstance(unwrap_vec_normalize(env), VecNormalize)
 
@@ -433,6 +433,17 @@ def test_sync_vec_normalize(make_env):
     assert allclose(obs, eval_env.normalize_obs(original_obs))
     assert allclose(env.normalize_reward(dummy_rewards), eval_env.normalize_reward(dummy_rewards))
 
+    # Check synchronization when only reward is normalized
+    env = VecNormalize(original_env, norm_obs=False, norm_reward=True, clip_reward=100.0)
+    eval_env = DummyVecEnv([make_env])
+    eval_env = VecNormalize(eval_env, training=False, norm_obs=False, norm_reward=False)
+    env.reset()
+    env.step([env.action_space.sample()])
+    assert not np.allclose(env.ret_rms.mean, eval_env.ret_rms.mean)
+    sync_envs_normalization(env, eval_env)
+    assert np.allclose(env.ret_rms.mean, eval_env.ret_rms.mean)
+    assert np.allclose(env.ret_rms.var, eval_env.ret_rms.var)
+
 
 def test_discrete_obs():
     with pytest.raises(ValueError, match=".*only supports.*"):