Django Admin. Make custom fields from custom form in ModelForm readonly.

[Maxven SkiLaren]

I have such admin code:

class SomeModelAdminForm(forms.ModelForm):
    custom_field1 = forms.CharField(...)
    custom_field2 = forms.IntegerField(...)


class SomeModelAdmin(admin.ModelAdmin):

    fields = ...
    readonly_fields = ...

    form = SomeModelAdminForm

    def get_readonly_fields(self, request, obj=None):
        ro = super(ProductAdmin, self).get_readonly_fields(request, obj)
        if obj:
            if obj.some_field is None:
                ro.append('custom_field1')
            else:
                ro.append('custom_field2')
        return ro

When I access the change form page of model, I am getting error

Unable to lookup 'custom_field1' on SomeModel or SomeModelAdmin or SomeModelForm

Is it a bug or I expect wrong behaviour from this code?