Почему не срабатывает is_valid для ModelForm при передаче instance?

Question

[V-ampre]

Переопределяю модель пользователя:

# models.py

class CustomConfirmedUser(AbstractBaseUser, PermissionsMixin):
    """Модель юзера с опцией подтверждени по имеил, наследуется от AbstractBaseUser"""

    email = models.EmailField("Email пользователя", max_length=128, unique=True)
    username = models.CharField("Логин пользователя", max_length=128, unique=True)
    is_active = models.BooleanField(default=True)
    is_staff = models.BooleanField(default=False)
    confirm = models.BooleanField("Пользователь подтвержден", default=False)

    objects = CustomUserManager()
    confirmed = CustomConfirmedManager()

    USERNAME_FIELD = 'username'
    REQUIRED_FIELDS = ['email',]

    def get_full_name(self):
        return self.username

    def get_short_name(self):
        return self.username

# forms.py

class CustomConfirmedUserChangeForm(forms.ModelForm):
    """Форма для редактирования CustomConfirmedUser"""
    password = ReadOnlyPasswordHashField()

    class Meta:
        model = CustomConfirmedUser
        fields = ('username', 'email', 'confirm', 'password', 'is_staff', 'is_superuser')

    def clean_password(self):
        return self.initial['password']

# tests.py

class TestCustomUserChangeForm(TestCase):

    def setUp(self):
        self.username = 'test'
        self.email = '[e-mail]'
        self.password = 'password123'
        self.user = CustomConfirmedUser.objects.create_user(
            self.username,
            self.email,
            self.password
        )

    def test_clean_password(self):
        form = CustomConfirmedUserChangeForm(
            {'username': 'newtest',},
            instance=self.user
        )
        form.is_valid()
        print(form.initial)
        print(form.errors)
        self.assertTrue(form.is_valid())

В итоге получаю:

{'password': 'pbkdf2_sha256$150000$nGiR5RBeDrAU$7s+IaSLRR3W/io1W7jQahjLRBGitxOwmvDXtEeD83/A=', 'is_superuser': False, 'email': '[e-mail]', 'username': 'test', 'is_staff': False, 'confirm': False}
<ul class="errorlist"><li>email<ul class="errorlist"><li>Это поле обязательно.</li></ul></li></ul>
F...
======================================================================
FAIL: test_clean_password (my_auth.tests.test_forms.TestCustomUserChangeForm)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/v-ampire/Projects/django-base/django_base/my_auth/tests/test_forms.py", line 100, in test_clean_password
    self.assertTrue(form.is_valid())
AssertionError: False is not true

Не понимаю, предано ж instance, разве username не должно браться оттуда?