mfogel/django-timezone-field — pull request #1 by mpdaugherty (2012-05-28 11:08:07+00:00)
Title: pytz.utz is not an instance of pytz.tzinfo.BaseTzInfo

Therefore, if you do something like:

>>> mymodel.timezone = 'UTC'
>>> mymodel.save()

You would get an exception saying that mymodel.timezone cannot be null in the DB.

I fixed this by also allowing instances of the UTC class in to_python() and get_prep_value()