Model Object fails to be added

[mekicha]

In the _add_items method in fields: https://github.com/jazzband/django-sortedm2m/blob/master/sortedm2m/fields.py#L83 There is a check if the object being added is an instance of django.db.models.Model class.

I find that adding a single instance fails that test as the object is indeed an instance of the Model class but passses when I add a queryset.

I am not totally sure what's the expected behaviour here.

Happy to provide more details