UserCreationForm issues with custom user model

forms.py

from django.contrib.auth.forms import UserCreationForm

class SignupForm(UserCreationForm):
    pass

gistfile1.pytb

>>> from myapp.forms import SignupForm
>>> f = SignupForm({'username': 'foo', 'password1': 'bar', 'password2': 'bar'})
>>> f.is_valid()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/path/to/my/venv/lib/python2.7/site-packages/django/forms/forms.py", line 126, in is_valid
    return self.is_bound and not bool(self.errors)
  File "/path/to/my/venv/lib/python2.7/site-packages/django/forms/forms.py", line 117, in _get_errors
    self.full_clean()
  File "/path/to/my/venv/lib/python2.7/site-packages/django/forms/forms.py", line 272, in full_clean
    self._clean_fields()
  File "/path/to/my/venv/lib/python2.7/site-packages/django/forms/forms.py", line 290, in _clean_fields
    value = getattr(self, 'clean_%s' % name)()
  File "/path/to/my/venv/lib/python2.7/site-packages/django/contrib/auth/forms.py", line 87, in clean_username
    User.objects.get(username=username)
  File "/path/to/my/venv/lib/python2.7/site-packages/django/db/models/manager.py", line 256, in __get__
    self.model._meta.object_name, self.model._meta.swapped
AttributeError: Manager isn't available; User has been swapped for 'myapp.User'

models.py

from django.contrib.auth.models import AbstractUser

class User(AbstractUser):
    pass

settings.py

AUTH_USER_MODEL = 'myapp.User'