diff --git a/includes/module.inc b/includes/module.inc
index fe2a980..5a04382 100644
--- a/includes/module.inc
+++ b/includes/module.inc
@@ -676,17 +676,17 @@ function module_hook($module, $hook) {
 /**
  * Determines which modules are implementing a hook.
  *
- * @param $hook
+ * @param string $hook
  *   The name of the hook (e.g. "help" or "menu").
- * @param $sort
+ * @param bool $sort
  *   By default, modules are ordered by weight and filename, settings this option
  *   to TRUE, module list will be ordered by module name.
- * @param $reset
+ * @param bool $reset
  *   For internal use only: Whether to force the stored list of hook
  *   implementations to be regenerated (such as after enabling a new module,
  *   before processing hook_enable).
  *
- * @return
+ * @return string[]|null
  *   An array with the names of the modules which are implementing this hook.
  *
  * @see module_implements_write_cache()
@@ -696,8 +696,10 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
   static $drupal_static_fast;
   if (!isset($drupal_static_fast)) {
     $drupal_static_fast['implementations'] = &drupal_static(__FUNCTION__);
+    $drupal_static_fast['verified'] = &drupal_static(__FUNCTION__ . '.verified');
   }
   $implementations = &$drupal_static_fast['implementations'];
+  $verified = &$drupal_static_fast['verified'];
 
   // We maintain a persistent cache of hook implementations in addition to the
   // static cache to avoid looping through every module and every hook on each
@@ -711,11 +713,12 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
   // per request.
   if ($reset) {
     $implementations = array();
+    $verified = array();
     cache_set('module_implements', array(), 'cache_bootstrap');
     drupal_static_reset('module_hook_info');
     drupal_static_reset('drupal_alter');
     cache_clear_all('hook_info', 'cache_bootstrap');
-    return;
+    return NULL;
   }
 
   // Fetch implementations from cache.
@@ -733,6 +736,7 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
     // The hook is not cached, so ensure that whether or not it has
     // implementations, that the cache is updated at the end of the request.
     $implementations['#write_cache'] = TRUE;
+    // Discover implementations for this hook.
     $hook_info = module_hook_info();
     $implementations[$hook] = array();
     $list = module_list(FALSE, FALSE, $sort);
@@ -750,7 +754,14 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
       drupal_alter('module_implements', $implementations[$hook], $hook);
     }
   }
-  else {
+
+  // The first time a hook is used in a request, the implementations must be
+  // "verified" with function_exists(), and the file specified by $group must be
+  // included.
+  // This applies both to implementations loaded from cache, and implementations
+  // that were discovered just now and may have been changed with
+  // hook_module_implements_alter().
+  if (empty($verified[$hook])) {
     foreach ($implementations[$hook] as $module => $group) {
       // If this hook implementation is stored in a lazy-loaded file, so include
       // that file first.
@@ -769,6 +780,7 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
         $implementations['#write_cache'] = TRUE;
       }
     }
+    $verified[$hook] = TRUE;
   }
 
   return array_keys($implementations[$hook]);