diff --git a/stable_baselines3/common/pytree_dataclass.py b/stable_baselines3/common/pytree_dataclass.py
index a8a0f4807..8f5306f25 100644
--- a/stable_baselines3/common/pytree_dataclass.py
+++ b/stable_baselines3/common/pytree_dataclass.py
@@ -84,9 +84,8 @@ def __new__(mcs, name, bases, namespace, slots=True, **kwargs):
                 # Otherwise we just mark the current class as what we're registering.
                 if not issubclass(cls, (FrozenPyTreeDataclass, MutablePyTreeDataclass)):
                     raise TypeError(f"Dataclass {cls} should inherit from FrozenPyTreeDataclass or MutablePyTreeDataclass")
-                mcs.currently_registering = cls
-        else:
-            mcs.currently_registering = cls
+
+        mcs.currently_registering = cls
 
         if name in _RESERVED_NAMES:
             if not (
@@ -105,10 +104,10 @@ def __new__(mcs, name, bases, namespace, slots=True, **kwargs):
 
             frozen = issubclass(cls, FrozenPyTreeDataclass)
             if frozen:
-                if not (not issubclass(cls, MutablePyTreeDataclass) and issubclass(cls, FrozenPyTreeDataclass)):
+                if issubclass(cls, MutablePyTreeDataclass) or not issubclass(cls, FrozenPyTreeDataclass):
                     raise TypeError(f"Frozen dataclass {cls} should inherit from FrozenPyTreeDataclass")
             else:
-                if not (issubclass(cls, MutablePyTreeDataclass) and not issubclass(cls, FrozenPyTreeDataclass)):
+                if not issubclass(cls, MutablePyTreeDataclass) or issubclass(cls, FrozenPyTreeDataclass):
                     raise TypeError(f"Mutable dataclass {cls} should inherit from MutablePyTreeDataclass")
 
             # Calling `dataclasses.dataclass` here, with slots, is what triggers the EARLY RETURN path above.