auctex11.5 ond GNU emacs21.4 doesn't query for master file

[QSPDVOFMGTUJ]

Hi,
I want to be prompted for which master file to use once per latex file,
but auctex never asks. First I tried putting the following in my
.emacs:
(setq-default TeX-master nil)

Then I tried to use "Customize AucTex", which wrote to my .emacs:
(custom-set-variables
  ;; custom-set-variables was added by Custom -- don't edit or
cut/paste it!
  ;; Your init file should contain only one such instance.
 '(TeX-master nil))

Both doesn't make difference. Can someone help me?

Jan