I dwa model:

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)

kiedy synchronizuję i tworzę użytkownika admin:

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

Jak mogę naprawić ten błąd?