which-func-mode keeps disappearing on the mode line

[Maverick Woo]

Hi,

I am using the vanilla emacs distribution from 
http://ftp.gnu.org/pub/gnu/emacs/windows/.

  GNU Emacs 23.2.1 (i386-mingw-nt6.1.7600)
   of 2010-05-08 on G41R2F1

I am trying to set up which-func-mode using its cleanup function as
follows. However, it keeps disappearing from the mode line after a
very brief blink (that shows the correct output). This is conducted
using "emacs -q" and so I don't think it's due to my other stuff. Any
clue?

(defun trim-front-spaces (s)
  (substring-no-properties s (string-match-p "[:space:]" s)))
(setq which-func-cleanup-function 'trim-front-spaces)
(which-function-mode 1)

P.S. In fact, even a simple cleanup function like (defun trim-front-
spaces (s) (concat s "a")) would give the same behavior.

Thanks!