Index: modules/system/system.api.php
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.api.php,v
retrieving revision 1.213
diff -u -p -r1.213 system.api.php
--- modules/system/system.api.php	14 Nov 2010 22:07:57 -0000	1.213
+++ modules/system/system.api.php	14 Nov 2010 22:59:09 -0000
@@ -862,6 +862,8 @@ function hook_page_build(&$page) {
  * hook_menu() implementations return an associative array whose keys define
  * paths and whose values are an associative array of properties for each
  * path. (The complete list of properties is in the return value section below.)
+ * Note that values in the array should never be set explicitly to NULL. Leave
+ * those fields empty instead or set them to FALSE or to an empty string.
  *
  * The definition for each path may include a page callback function, which is
  * invoked when the registered path is requested. If there is no other