Index: includes/module.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/module.inc,v
retrieving revision 1.180
diff -u -r1.180 module.inc
--- includes/module.inc	28 Jan 2010 13:56:24 -0000	1.180
+++ includes/module.inc	15 Feb 2010 17:02:04 -0000
@@ -338,6 +338,8 @@ function module_enable() {
 
   $invoke_modules = array();
 
+  // Required for _drupal_install_module().
+  include_once DRUPAL_ROOT . '/includes/install.inc';
   // Try to install the enabled modules and collect which were installed.
   // $module_list is not changed and already installed modules are ignored.
   $modules_installed = array_filter($module_list, '_drupal_install_module');