我知道从Django 1.7开始，我不需要使用South或任何其他迁移系统，所以我只是使用简单的命令 python manage.py makemigrations

但是，我得到的只是这个错误：

You are trying to add a non-nullable field 'new_field' to userprofile without a default;
we can't do that (the database needs something to populate existing rows).

这是models.py：

class UserProfile(models.Model):
    user = models.OneToOneField(User)
    website = models.URLField(blank=True)
    new_field = models.CharField(max_length=140)

有哪些选择？