y-or-n-p-with-timeout fails when using dialog box input

[Järneström Jonas]

In GNU Emacs 20.7.1 (sparc-sun-solaris2.8, X toolkit)
 of Tue Jan 16 2001 on sunray8.era-a.ericsson.se
configured using `configure  --prefix=/usr/bag/emacs/20.7 
--datadir=/usr/bag/emacs/share'

See the funs below. 
When I run the first one, timeout never happens.
When I run the second one, timeout works ok.
Why dont I get the timeout when using the dialog box input method?

Thanks,
Jonas Jarnestrom

(defun dialog-box-timeout-never-happens ()
  (let* ((last-nonmenu-event nil))
    (y-or-n-p-with-timeout "prompt" 3 'timeout)))

(defun minibuffer-timeout-works ()
  (let* ((foo nil))
    (y-or-n-p-with-timeout "prompt" 3 'timeout)))