Disabled ModelMultipleChoiceField does not validate anymore

[karyon]

i have the following test:

    def test_foo(self):
        # Course has an m2m to degrees
        course = mommy.make(Course, semester=mommy.make(Semester), degrees=[mommy.make(Degree)])

        # get_form_data_from_instance creates a dict with all the attributes
        form_data = get_form_data_from_instance(ContributorCourseForm, course)

        # ContributorCourseForm is a simple ModelForm on Course
        form = ContributorCourseForm(form_data, instance=course)
        self.assertTrue(form.is_valid())
        
        # recreate form
        form = ContributorCourseForm(form_data, instance=course)
        # disable degrees field
        form.fields['degrees'].disabled = True
        self.assertTrue(form.is_valid())

the second assert fails. form.errors says "degrees: Enter a list of values." which doesn't make sense, since 1. it does have a degree (the form renders correctly with that degree selected), and 2. this shouldn't happen just because the field is disabled.

this showed up in 3744fc1666c41aeb4ed44f31605bafdb413b42bc, the fix for https://code.djangoproject.com/ticket/26917#comment:1

likely related: https://code.djangoproject.com/ticket/25980, same error message when providing a queryset as initial value to a disabled ModelMultipleChoiceField.