Form validator cannot pass ModelMultipleChoiceField by checkbox

[cloudy-sfu]

In the form, I set:

fill_na_avg = forms.ModelMultipleChoiceField(Column.objects.all(), widget=forms.HiddenInput(), required=False)

In the HTML file, I write a group of :

<input type="checkbox" name="fill_na_avg" value="{{ column.id }}">

After submitting the POST, there is:

<QueryDict: {'fill_na_avg': ['41', '42', '43']}>

It seems normal but when checking it by the "is_valid" function, it reports:

fill_na_avg
Enter a list of values.

If I remove widget setting like:

fill_na_avg = forms.ModelMultipleChoiceField(Column.objects.all(), required=False)

It works. So, why widget affects the validation of forms?