Почему не определяется свойство через widget?

class VendorRegForm(forms.ModelForm):
    class Meta:
        model = VendorProfile
        fields = ('org_name','slug', 'kind_activity')
        ....
       kind_activity = forms.ModelChoiceField(queryset = Activity.objects.all(),
                                                               widget=forms.Select(attrs={'class': 'form-control'}))