IntegrityError：“city_id”列中的空值违反非空约束

I two model:

我两个型号：

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)
class City(models.Model