Django forms won't clean a second time.

django-wtf.py

>>> from django.contrib.auth.forms import UserCreationForm
>>> form = UserCreationForm({'username': 'admin123', 'password1': 's3cr3t', 'password2': 's3cr3t'})
>>> form.is_valid()
True
>>> form.data['username'] = ''
>>> form.is_valid()
True
>>> form._errors = None
>>> form.is_valid()
False
>>> form.errors
{'username': [u'This field is required.']}
>>>