diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -3113,10 +3113,6 @@
     $loader->registerPrefixes($prefixes);
     $loader->registerNamespaces($namespaces);
 
-    if (!interface_exists('SessionHandlerInterface')) {
-      $loader->registerPrefixFallback($namespaces['SessionHandlerInterface']);
-    }
-
     // Register the loader with PHP.
     $loader->register();
   }
@@ -3156,6 +3152,10 @@
     $loader->registerPrefixes($prefixes);
     $loader->registerNamespaces($namespaces);
 
+    if (!interface_exists('SessionHandlerInterface', FALSE)) {
+      $loader->registerPrefix('SessionHandlerInterface', DRUPAL_ROOT . '/vendor/symfony/http-foundation/Symfony/Component/HttpFoundation/Resources/stubs/SessionHandlerInterface.php');
+    }
+
     // Register the loader with PHP.
     $loader->register();
   }