class User(AbstractBaseUser,PermissionsMixin):

    first_name = models.CharField(_('first name'),max_length=20)
    last_name = models.CharField(_('last name'),max_length=20)
    user_type = models.CharField(_('user type'),max_length=10,choices=TYPE,default='Seller')
    paypal_email = models.EmailField(_('paypal email'),max_length=255,unique=True,null=True,blank = True)
    company_name = models.CharField(_('company name'),max_length=20)
    receive_email = models.BooleanField(_('receive email'),default=False)
    email = models.EmailField(_('email address'),max_length=100,unique=True)
    is_active = models.BooleanField(_('is active'),default=True)
    date_joined = models.DateTimeField(_('date joined'),default=timezone.now)
    is_superuser = models.BooleanField(default=False)
    is_staff = models.BooleanField(default=False)

    username_FIELD = 'email'

    objects = CustomaccountManager()

    def __str__(self):
        return str(self.email)

如何在Django中编写用户模型测试单元，我正在尝试编写此错误信息，请帮助我解决此问题