Problem with asm-mode-hook

[Anders K.]

I'm trying to change the assembler mode comment character from ; to #, 
so I put this at the end of ~/.emacs:

(defun my-asm-mode-hook ()
  (setq asm-comment-char ?#))
(add-hook 'asm-mode-hook 'my-asm-mode-hook)

Unfortunately, the change only takes effect starting with the second 
assembly file I open. Anyone know why? (I have a similar 
c-mode-common-hook that works perfectly.)

Anders