Emacs: set and toggle show-trailing-whitespace

Two related questions using emacs 23.3.1 on linux:

First, why can't I set the value of show-trailing-whitespace to t with setq as shown below? When I put the setq version in my .emacs it does not change the value (as seen functionally and by using M-x describe-variable).

(setq show-trailing-whitespace t)  ; Does not change variable value or give error

(custom-set-variables              ; Sets show-trailing-whitespace as expected
 '(show-trailing-whitespace t))

Second, how can I toggle the value between t and nil? I thought that this answer was exactly what I needed, but it doesn't work in this case. I used:

(global-set-key "\M-ow" 'tf-toggle-show-trailing-whitespace)

(defun tf-toggle-show-trailing-whitespace ()
    "Toggle show-trailing-whitespace between t and nil"
    (interactive)
    (setq show-trailing-whitespace (if (= show-trailing-whitespace nil) t nil))
    (redraw-display))

When I hit M-ow I get an error Wront type argument: number-or-marker-p, nil. Any ideas?