我得到错误django模型像这样，当我试图makemigrations：

You are trying to add a non-nullable field 'person' to owner without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
 1) Provide a one-off default now (will be set on all existing rows)
 2) Quit, and let me add a default in models.py

我使用的是django 1.8，这是我的模型：

class Person(models.Model):
    user            = models.OneToOneField(User)
    alphanumeric    = RegexValidator(r'^[0-9a-zA-Z]*$', message='hanya yang mengandung karakter alphanumeric')
    email           = models.EmailField(verbose_name='email address', unique=True, max_length=244)
    username        = models.CharField(unique=True, max_length=20, validators=[alphanumeric])
    first_name      = models.CharField(max_length=30, null=True, blank=True)
    last_name       = models.CharField(max_length=30, null=True, blank=True)
    date_of_birth   = models.DateTimeField()
    date_joined     = models.DateTimeField(auto_now_add=True)

    USERNAME_FIELD      = 'username'
    REQUIRED_FIELDS     = ['email']

    def get_full_name(self):
        fullname = self.first_name+" "+self.last_name
        return self.fullname

    def get_short_name(self):
        return self.username

    def list_operator(self):
        return self.operators.all()

    def __str__(self):
        return self.email

class Operator(models.Model):
    person          = models.ForeignKey(Person, related_name="operators", null=True)
    alphanumeric    = RegexValidator(r'^[0-9a-zA-Z]*$', message='hanya yang mengandung karakter alphanumeric')
    email           = models.EmailField(verbose_name='email address', unique=True, max_length=244)
    username        = models.CharField(unique=True, max_length=20, validators=[alphanumeric])
    first_name      = models.CharField(max_length=30, null=True, blank=True)
    last_name       = models.CharField(max_length=30, null=True, blank=True)
    date_of_birth   = models.DateTimeField(auto_now_add=True)

    def __str__(self):
        return self.username;

我想知道我的代码哪里错了。你能帮我解决这个问题吗？