invalid choice error when deleting a TabbedInline in Django 1.5

save.py

    def save_existing_objects(self, commit=True):
        self.changed_objects = []
        self.deleted_objects = []
        if not self.initial_forms:
            return []

        saved_instances = []
        try:
            forms_to_delete = self.deleted_forms
        except AttributeError:
            forms_to_delete = []
        for form in self.initial_forms:
            pk_name = self._pk_field.name
            raw_pk_value = form._raw_value(pk_name)

            # clean() for different types of PK fields can sometimes return
            # the model instance, and sometimes the PK. Handle either.
            try:
                pk_value = form.fields[pk_name].clean(raw_pk_value)
                pk_value = getattr(pk_value, 'pk', pk_value)

                obj = self._existing_object(pk_value)
                if form in forms_to_delete:
                    self.deleted_objects.append(obj)
                    obj.delete()
                    continue
                if form.has_changed():
                    self.changed_objects.append((obj, form.changed_data))
                    saved_instances.append(self.save_existing(form, obj, commit=commit))
                    if not commit:
                        self.saved_forms.append(form)
            except ValidationError:
                pass
        return saved_instances