diff --git a/trytond/trytond/pool.py b/trytond/trytond/pool.py
index 5b0b7932cc..6507ca9232 100644
--- a/trytond/trytond/pool.py
+++ b/trytond/trytond/pool.py
@@ -218,11 +218,10 @@ class Pool(object):
                     cls = resolve(qualname)
                     assert issubclass(cls.__class__, PoolMeta)
                 try:
-                    previous_cls = self.get(cls.__name__, type=type_)
-                    cls = type(
-                        cls.__name__, (cls, previous_cls), {'__slots__': ()})
+                    base_classes = (cls, self.get(cls.__name__, type=type_))
                 except KeyError:
-                    pass
+                    base_classes = (cls,)
+                cls = type(cls.__name__, base_classes, {'__slots__': ()})
                 assert issubclass(cls, PoolBase), (
                     f"{cls} is not a subclass of {PoolBase}")
                 self.add(cls, type=type_)