diff --git a/core/lib/Drupal/Component/Plugin/PluginBag.php b/core/lib/Drupal/Component/Plugin/PluginBag.php
index d6efc6b..a62047d 100644
--- a/core/lib/Drupal/Component/Plugin/PluginBag.php
+++ b/core/lib/Drupal/Component/Plugin/PluginBag.php
@@ -200,7 +200,7 @@ public function count() {
   public function __sleep() {
     // Do not serialize instantiated plugins.
     $this->pluginInstances = array();
-    return get_object_vars($this);
+    return array_keys(get_object_vars($this));
   }
 
 }