All tests fail with TypeError: isinstance() arg 2 must be a type or tuple of types

[Sceva]

The full error (last on in the stack) is 

File "/home//venv/lib/python3.3/site-packages/django/db/models/fields/related.py", line 1699, in get_default
    if isinstance(field_default, self.rel.to):
TypeError: isinstance() arg 2 must be a type or tuple of types

Python 3.3.2, Django 1.7b1

If I switch to 1.7b4, the error changes to 

  File "/home/s/Pycharm/flf/venv/lib/python3.3/site-packages/django/db/migrations/state.py", line 86, in render
    model=lookup_model,
ValueError: Lookup failed for model referenced by field study.Card.image: studyimages.StudyImage

The error occurs when I create a FK field to a model in another app:

study/models.py:

....

    image = models.ForeignKey('studyimages.StudyImage', default=None, blank=True, null=True, related_name='cardpic')

The initial problem occurred with migrate, which I overcame by changing the fk model to one in the same app, migrating, then changing the fk back to the above the migrate command is successful.

I can run my unittests using django 1.6.5, and my functional tests may run if I stop using StaticLiveServerCase.

I have seen several bugs about migrated and unmigrated apps not able to use FKs, but both of my apps have migrations before introducing the FK.  Thanks in advance.

[Sceva]

The error appears when I run tests and it is trying to create the default database. 

I forgot to add that I found this, https://github.com/pelme/pytest_django/issues/68 that seems to be a similar issue.

[Sceva]

Deleted all the migration files, re-ran makemigrations and migrate, and errors disappeared.