TypeError: 'NoneType' object is not callable

class MyUserManager(BaseUserManager):

def create_user(self, email, password=None,
                first_name=None,
                last_name=None
               ):
    now = timezone.now()
    if not email:
        raise ValueError('The given email must be set')
    email = self.normalize_email(email)

    user = self.model(email=email,
                        first_name=first_name,
                        last_name=last_name
                      )

    user.set_password(password)
    user.save(using=self._db)
    return user

class MyUser(AbstractBaseUser, PermissionsMixin):

email = models.EmailField(_('email address'), max_length=254, unique=True)
first_name = models.CharField(_('first name'), max_length=30, blank=False)
last_name = models.CharField(_('last name'), max_length=30, blank=False)

objects = MyUserManager()

USERNAME_FIELD = 'email'
REQUIRED_FIELDS = []

class Meta:
    verbose_name = _('user')
    verbose_name_plural = _('users')

def get_full_name(self):
    """
    Returns the first_name plus the last_name, with a space in between.
    """
    full_name = '%s %s' % (self.first_name, self.last_name)
    return full_name.strip()