kill-region defadvice

[Joe]

Hey all,

I liked the tip in Steve Yegge's "Effective Emacs" of rebinding
backward-kill-word to C-w but I still wanted to use C-w for kill-
region.  Originally I did this by writing a new function that checked
if the mark was active. However, I think that using defadvice is a
better solution but I got stuck while trying to write it.  Here's my
attempt:

(defadvice kill-region (around smart-kill activate)
  "If something is highlighted, then kill the region, otherwise
backward-kill-word"
  (interactive (list (point) (mark)))
   (if mark-active
       ad-do-it
     (backward-kill-word 1)))

When I run the command it works normally if I have something
highlighted, but if I don't have anything highlighted, I get the usual
"The mark is not active now.".

How can I kill the region if the mark is active and backward-kill-word
if its not?

Thanks for the help,
Joe