我两个模型：

class City(models.Model):
    name = models.CharField(max_length=50)
    country = models.OneToOneField(Country)

    def __unicode__(self):
        return self.name


class UserProfile(models.Model):
    user = models.OneToOneField(User)
    city = models.OneToOneField(City)

当我同步数据库并创建管理员用户时：

IntegrityError: null value in column "city_id" violates not-null constraint

如何解决此错误？