diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
index a2809de0dd..d607c27814 100644
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -345,8 +345,7 @@ class AutoField(Field):
         return Field.get_manipulator_new_data(self, new_data, rel)
 
     def contribute_to_class(self, cls, name):
-        if cls._meta.has_auto_field:
-            raise AssertionError, "A model can't have more than one AutoField."
+        assert not cls._meta.has_auto_field, "A model can't have more than one AutoField."
         super(AutoField, self).contribute_to_class(cls, name)
         cls._meta.has_auto_field = True