bug#8256: widget-complete does not complete for type 'directory on w32

[Lennart Borgman]

On Tue, Mar 15, 2011 at 6:18 PM, Stefan Monnier
<monnier@iro.umontreal.ca> wrote:
>>   emacs -Q
>>   Paste to *scratch* this code:
>
>>       (defcustom my-dir nil
>>           "doc"
>>          :type 'directory)
>
>>   Eval it.
>>   M-x customize-option RET my-dir RET
>
> Problem here: nil is not a directory, so the above defcustom is not
> valid.  Oddly enough Customize does not signal the error, and instead
> gets into a weird state.  Once you set my-dir to a string, things work
> much better.

What should signal that it the defcustom is not valid? And when?