drupal menu_alter looking for include file in the wrong place

I currently have a simple menu_alter hook in a new module I'm building which allows simple access control for menus:

<?php 

function amh_menu_menu_alter(&$items)
{
    $items['admin/build/menu/access'] = array(
    'title' => 'Access',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('amh_menu_access_configure'),
    'access arguments' => array('administer menu'),
    'type' => MENU_LOCAL_TASK,
    'weight' => 5,
    'file' => 'amh_menu.admin.inc',
  );
}

However, I get the following error when loading the page:

[Mon Feb 21 11:25:15 2011] [error] [client 10.2.2.106] PHP Fatal error:  require_once(): 
Failed opening required '/amh_menu.admin.inc' 
(include_path='.:/usr/share/php:/usr/share/pear') 
in /var/www/shop.dev.amh/www/includes/menu.inc on line 346, 
referer: http://shop.dev.amh/admin/build/menu

Looking at other modules which have menu and menu_alter hooks, they all refer to local module.admin.inc in places, and they don't have this problem and don't appear to specify any extra path.

Indeed - the documentation for menu hooks says that the "file path" menu item parameter i开发者_开发知识库s

The path to the directory containing the file specified in "file". This defaults to the path to the module implementing the hook.

How do I make this work?

You can explicitly set the file path to drupal_get_path('module', 'yourmodule'). That is the default, but the default is set before hook_menu_alter() is called.

The question is, why do you define a new menu item in hook_menu_alter() instead of hook_menu(). The alter hook is supposed to change existing hooks of other modules, not add new ones.