'getattr(): attribute name must be string' error in admin panel for a model with an ImageField

I have the following model set up:

class UserProfile(models.Model):
    "Additional attributes for users."
    url = models.URLField()
    location = models.CharField(max_length=100)
    user = models.ForeignKey(User, unique=True)
    avatar = models.ImageField(upload_to='/home/something/www/avatars', height_field=80, width_field=80)

    def __unicode__(self):
        return "Profile of " + self.user.username

It's supposed to store additional information about a user, for example an avatar.

Unfortunately, when I try to upload an image via the admin panel, it gives me an error, something like:

getattr(): attribute name must be string

Which is not produced when I remove that field from the model, do a db reset and reload the server. I'd imagine the cause is this particular field, just not sure how.

This is my traceback:

File "/usr/lib/pymodules/python2.6/django/core/handlers/base.py" in get_response
  92.                 response = callback(request, *callback_args, **callback_kwargs)
File "/usr/lib/pymodules/python2.6/django/contrib/admin/options.py" in wrapper
  226.                 return self.admin_site.admin_view(view)(*args, **kwargs)
File "/usr/lib/pymodules/python2.6/django/views/decorators/cache.py" in _wrapped_view_func
  44.         response = view_func(request, *args, **kwargs)
File "/usr/lib/pymodules/python2.6/django/contrib/admin/sites.py" in inner
  186.             return view(request, *args, **kwargs)
File "/usr/lib/pymodules/python2.6/django/db/transaction.py" in _commit_on_success
  240.                 res = func(*args, **kw)
File "/usr/lib/pymodules/python2.6/django/contrib/admin/options.py" in add_view
  718.                 new_object = self.save_form(request, form, change=False)
File "/usr/lib/pymodules/python2.6/django/contrib/admin/options.py" in save_form
  551.         return form.save(commit=False)
File "/usr/lib/pymodul