parent: [ 'Invalid pk "0" - object does not exist.' ] and null or blank value not store in mysql

class AccountType(ModelBase):
    parent = models.ForeignKey('self',on_delete = models.SET_NULL, null=True, blank=True, default=None )

class AccountTypeSerializer(serializers.ModelSerializer):
    parent = serializers.PrimaryKeyRelatedField(queryset=AccountType.objects.all(), required=False,null=True, blank=True)
    class Meta:
        model = AccountType
        fields = ('id','parent','account_type_name')

how to set null and empty value in parent field

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