Django test client response context None

I have moved my Django app from my development machine (OS X, Python 2.6.5, Django 1.2.3) to a staging server (Ubuntu VM, Python 2.6.6, Django 1.2.3).

If I now run my test suite on the staging server, two tests fail when using the Django TestClient because response.context is None (but response.content is correct).

For example:

self.assertEquals(self.session.pk, response.context['db_session'].pk)

These test cases pass on the development machine.

Has anybody encountered similar problems?