Because this assertion https://github.com/django/django/blob/master/django/db/models/fields/__init__.py#L706 was replaced with this system check https://github.com/chrismedrela/django/blob/gsoc2013-checks/django/db/models/fields/__init__.py#L859. There is a test too https://github.com/chrismedrela/django/blob/gsoc2013-checks/tests/invalid_models/test_ordinary_fields.py#L21.