添加自定义用户模型后无法创建超级用户
我创建了一个叫 CustomUser 的类，它是从 AbstractUser 这个类扩展出来的，并且我添加了一个外键字段，用来关联 City 模型。
class City(models.Model):
created_dt = models.DateTimeField("Created Time", auto_now_add=True)
code = models.CharField(max_length=64, unique=True)
name = models.CharField(max_length=64)
class CustomUser(AbstractUser):
city = models.ForeignKey(City)
REQUIRED_FIELDS = ['city']
我把 CustomUser 加入到了设置里：
AUTH_USER_MODEL = "myapp.CustomUser"
当我尝试同步数据库时，它提示我需要创建一个超级用户。当我填写城市信息时，出现了这个错误：
City: 1
Traceback (most recent call last):
File "manage.py", line 10, in <module>
execute_from_command_line(sys.argv)
File "/var/www/uib_webservice/.virt1/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 399, in execute_from_command_line
utility.execute()
File "/var/www/uib_webservice/.virt1/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 392, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/var/www/uib_webservice/.virt1/local/lib/python2.7/site-packages/django/core/management/base.py", line 242, in run_from_argv
self.execute(*args, **options.__dict__)
File "/var/www/uib_webservice/.virt1/local/lib/python2.7/site-packages/django/core/management/base.py", line 285, in execute
output = self.handle(*args, **options)
File "/var/www/uib_webservice/.virt1/local/lib/python2.7/site-packages/django/contrib/auth/management/commands/createsuperuser.py", line 116, in handle
user_data[field_name] = field.clean(raw_value, None)
File "/var/www/uib_webservice/.virt1/local/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 255, in clean
self.validate(value, model_instance)
File "/var/www/uib_webservice/.virt1/local/lib/python2.7/site-packages/django/db/models/fields/related.py", line 1201, in validate
using = router.db_for_read(model_instance.__class__, instance=model_instance)
File "/var/www/uib_webservice/.virt1/local/lib/python2.7/site-packages/django/db/utils.py", line 250, in _route_db
return hints['instance']._state.db or DEFAULT_DB_ALIAS
AttributeError: 'NoneType' object has no attribute '_state'
有人能告诉我这是为什么吗？我在 Django 方面还是个新手，谢谢。