django-positions - multi-table model inheritance using parent_link

Using https://github.com/jpwatts/django-positions,

I have a few models that inherit from a parent one, for example:

class ContentItem(models.Model):

    class Meta:
        ordering = ['position']

    content_group = models.ForeignKey(ContentGroup)
    position      = PositionField(collection='content_group', parent_link='contentitem_ptr')

class Text(ContentItem):

    title = models.CharField(max_length=500, unique=False, null=True, blank=True)

Using https://github.com/jpwatts/django-positions,

I have a few models that inherit from a parent one, for example:

class ContentItem(models.Model):

    class Meta:
        ordering = ['position']

    content_group = models.ForeignKey(ContentGroup)
    position      = PositionField(collection='content_group', parent_link='contentitem_ptr')

class Text(ContentItem):

    title = models.CharField(max_length=500, unique=False, null=True, blank=True)

I understand I need to use the parent_link argument (here's the documentation). But I get this error when I use it:

websites.Text: (models.E015) 'ordering' refers to the non-existent field 'position'.

When using the parent_link argument it's as if the position field has been deleted out of the model completely. I've tried various field names such as contentitem_ptr_id (the actual name of the linking field), but no luck. Anything identifiable I'm doing wrong here?