Clever way to do this?

[Deniz Dogan]

I have a command like this:

(defun ftip-rate-movie (rating)
  (interactive
   (list
    (if (not ftip-movie-id)
        (error "Not viewing movie")
      (completing-read "Rating: " '("1" "2" "3" "4" "5" "Remove 
rating") nil t))))
  (unless ftip-movie-id
    (error "Not viewing movie"))

  ;; ...code here
  )

Is there any more intelligent way to do this?  I'm not too keen on the 
code duplication.

Thanks,
Deniz