Allow null in foreign key to user. Django

I have this model

class Vacancy(models.Model):
    user = models.ForeignKey(User, null=True, blank=True, default = None)
    name = models.CharField(max_length=64)

When in admin i try to creat a vacancy without a user. And it throws an error " club_vacancy.user_id may not be NULL". Am i doing something wrong?