guess-TeX-master bug?

I'm trying to use the guess-TeX-master function (from emacswiki auctex) but I get this error:

Wrong type argument: stringp, nil

My elisp knowledge is quite poor. Does somebody know the problem?

[EDIT]

This error was because I forgot do it the latex hook.

Anyway I trying to do something different. I want to find the file (i开发者_运维知识库n the same directory) that has the "begin{document}". Because my elisp skills are poor, I do it in a script:

In my .emacs:

(add-hook 'LaTeX-mode-hook  
  (lambda ()  
    (setq TeX-master (execvp "guess-TeX-master"  
      (file-name-directory (buffer-file-name))))))  

execvp defun is here: http://www.emacswiki.org/emacs/ExecuteExternalCommand

The guess-TeX-master script is:

grep "begin{document}" $1/*.tex | cut -d ':' -f 1 | tr -d '\n'

How to do the same using elisp only?

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Can you paste your code(at paste.lisp.org and share the link here) so we can find out if it really has syntactic issue.. Basically from what I understand you are trying to use a wrong argument for the operator...