HiddenInput ModelChoiceField Won't Validate

Question

I have a ModelChoiceField in my form that I want to hide. To do this, I'm setting the widget to forms.HiddenInput(). However, whenever the widget is set to be forms.HiddenInput(), the field will not validate. I get the validation error (Hidden field location_choices) Enter a list of values. When all my other code is the same, but I leave the ModelChoiceField widget as the default, the form validates fine.

Here is my code. The field that is not validating is location_choices:

class SingleSessionForm(forms.ModelForm):

    start_time = forms.DateTimeField(widget=SplitDateTimeWidget)
    end_time = forms.DateTimeField(widget=SplitDateTimeWidget)

    price = forms.IntegerField(
        label=_('Price'),
        min_value=0,
    )

    def __init__(self, *args, **kwargs):
        super(SingleSessionForm, self).__init__(*args, **kwargs)
        self.fields['price'].initial = self.instance.prices[0]
        if self.instance.event.studio:
            self.fields['location_choices'].widget = forms.HiddenInput()
            self.fields['location_choices'].required = False
            self.fields['house_calls'].widget = forms.HiddenInput()


        self.helper = FormHelper()
        self.helper.form_id = 'id-add_sessions'
        self.helper.form_class = 'blueForms'
        self.helper.layout = Layout(
            Fieldset(
                'Time',
                'start_time',
                'end_time',
            ),
            Fieldset(
                'Details',
                Field(
                    'location', 'location_choices', 'house_calls'
                ),
                'max_enrollment',
                Field(
                    PrependedAppendedText('price', '$', '.00'),
                )
            ),
            FormActions(
                Submit('action', 'Submit', css_class='btn btn-primary btn-large'),
                Button('cancel', 'Cancel', css_class='btn btn-danger btn-large', onclick="window.location.replace('" + self.instance.get_absolute_url() + "')")
            )
        )

    def clean_location_choices(self):
        if self.instance.event.studio:
            return self.instance.event.studio.locations
        return self.cleaned_data['location_choices']

    def clean_allow_home_training(self):
        if self.instance.event.studio:
            return False
        return self.cleaned_data['house_calls']