diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 2ddc3ee..5fe4287 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -975,7 +975,14 @@ protected function initializeSettings(Request $request) {
     if ($class_loader_class == get_class($this->classLoader)
         && Settings::get('class_loader_auto_detect', TRUE)
         && function_exists('apc_fetch')) {
-      $prefix = Settings::getApcuPrefix('class_loader', $this->root);
+      // We need a file path to a class which should be in the vendor directory,
+      // so that we can modify our APC identifier hash with it. We do this so
+      // that if someone moves the vendor directory, ApcClassLoader won't fail
+      // to load the classes contained within it. We use the path to the Request
+      // class because we know the classloader has already found it.
+      // @todo: Consider removing this after https://www.drupal.org/node/2380389
+      $request_class_path = $this->classLoader->findFile('Symfony\Component\HttpFoundation\Request');
+      $prefix = Settings::getApcuPrefix('class_loader', $this->root, $request_class_path);
       $apc_loader = new \Symfony\Component\ClassLoader\ApcClassLoader($prefix, $this->classLoader);
       $this->classLoader->unregister();
       $apc_loader->register();
diff --git a/core/lib/Drupal/Core/Site/Settings.php b/core/lib/Drupal/Core/Site/Settings.php
index 21298b4..5a972dc 100644
--- a/core/lib/Drupal/Core/Site/Settings.php
+++ b/core/lib/Drupal/Core/Site/Settings.php
@@ -161,9 +161,14 @@ public static function getHashSalt() {
    * module directories setting apcu_ensure_unique_prefix would allow the sites
    * to share APC cache items.
    *
-   * @param $identifier
+   * @param string $identifier
    *   An identifier for the prefix. For example, 'class_loader' or
    *   'cache_backend'.
+   * @param string $root
+   *   The root directory of the Drupal installation.
+   * @param string $site_path
+   *   (optional) The multisite site path, useful for SimpleTest. Can be an
+   *   arbitrary string as well. Defaults to an empty string.
    *
    * @return string
    *   The prefix for APC user cache keys.