hook_menu_alter() usage

Hi,

As I learnt in #178138, when writing my own content node type in a module (this case in 'faq'), I shouldn't define anything in hook_menu() for "node/add/faq". Instead it's automatically generated and I was advised to use hook_menu_alter() if I wanted to change anything for it. I want to change the access to "create faq". I'm trying the code below:

function faq_menu_alter(&$callbacks) {
  $callbacks['node/add/faq']['access callback'] = "user_access";
  $callbacks['node/add/faq']['access arguments'] = "create faq";
}

However, after adding in this function I get the error below on "node/add/faq":

warning: Invalid argument supplied for foreach() in /var/www/drupal_6.1_20070924/html/includes/menu.inc on line 258.

I've looked at the api docs which haven't really helped and I couldn't find any examples or useful references.

All help appreciated.

Thanks,
Stella