diff --git a/format.py b/format.py
index 3b9851f..65ebad7 100644
--- a/format.py
+++ b/format.py
@@ -52,7 +52,7 @@ def __init__(cls, name, bases, attrs):
         cls._context = set(popattr(cls, 'context_specs',
                            attrs.pop('context_specs', ())))
         # protocols merge: top-down through inheritance
-        for base in reversed(cls.__bases__):
+        for base in reversed(bases):
             cls._protocols.update(getattr(base, '_protocols', {}))
             cls._validators.update(getattr(base, '_validators', {}))
             cls._context.update(getattr(base, '_context', ()))