foreign key to self in sqllite3

I have model field foreign key to self
but sql have error and cant create null field for field
why?

class Category(models.Model):
    title = models.CharField(max_length=250,\
                             unique=True, verbose_name=_("Title"))
    slug = models.SlugField(max_length=100,\
                            verbose_name=_("Slug"),\
                            help_text=_("This Field will fill\
                            automaticaly by title"))
    parent = models.ForeignKey('self', blank=True, null=True, verbose_name=_("parent"))

thanks

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