Incompatibility with Django 1.5: AttributeError: 'QuerySet' object has no attribute '_pk_val'

Django's commit django/django@611c4d6f1c24763e5e6e331a5dcf9b610288aaa8 breaks django-sortedm2m:

    Traceback (most recent call last):
      File "sortedm2m/fields.py", line 180, in __set__
        manager.add(*value)
      File "django/db/models/fields/related.py", line 642, in add
        self._add_items(self.source_field_name, self.target_field_name, *objs)
      File "sortedm2m/fields.py", line 108, in _add_items
        source_field_name: self._pk_val,
      File "model_utils/managers.py", line 131, in __getattr__
        return getattr(self.get_query_set(), name)
    AttributeError: 'QuerySet' object has no attribute '_pk_val'

The commit message for django/django@611c4d6f1c24763e5e6e331a5dcf9b610288aaa8 is:

    commit 611c4d6f1c24763e5e6e331a5dcf9b610288aaa8
    Author: Anssi Kääriäinen <akaariai@gmail.com>
    Date:   Sun Oct 28 16:47:07 2012 +0200
    
        Fixed #18823 -- Ensured m2m.clear() works when using through+to_field
        
        There was a potential data-loss issue involved -- when clearing
        instance's m2m assignments it was possible some other instance's
        m2m data was deleted instead.
        
        This commit also improved None handling for to_field cases.