[dfv] field filters not executed for optional?

[Jiří P. <ji...@ge...>]

Hi,

what I want to achieve is to filter checkbox values so that they are 
either 1 or 0, ie no undef values.

So I put them into optional and set up field_filters for them with an 
sub like this

   sub checkbox_filter {
        return defined $_[0] && lc $_[0] eq 'on' ? 1 : 0;
}

But it looks like the filed filters are never executed.