NOT NULL constraint failed, а почему?

Когда я пытаюсь создать суперпользователя

python manage.py createsuperuser

я получаю следующую ошибку

django.db.utils.IntegrityError: NOT NULL constraint failed: me_teacher.subject_id

me/models

from django.conf import settings
from django.contrib.auth.models import User, AbstractUser
from django.db import models
import sys
sys.path.append('../')
from wisdom.models import Subject

class Teacher(AbstractUser):
    name = models.CharField(max_length=20)
    surname = models.CharField(max_length=20)
    user = models.OneToOneField(settings.AUTH_USER_MODEL,
        on_delete=models.CASCADE)
    subject = models.ForeignKey(Subject, related_name='teacher_subject',
        on_delete=models.CASCADE)

wisdom/models

from django.db import models

class Subject(models.Model):
    title = models.CharField(max_length=200)
    slug = models.SlugField(max_length=200, unique=True)

    class Meta:
        ordering = ['title']

    def __str__(self):
        return self.title

в чём моя проблема?