diff --git a/core/lib/Drupal/Core/Plugin/Discovery/HookDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/HookDiscovery.php
index 9a35498..37786be 100644
--- a/core/lib/Drupal/Core/Plugin/Discovery/HookDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/HookDiscovery.php
@@ -44,6 +44,7 @@ public function getDefinition($plugin_id) {
    * Implements Drupal\Component\Plugin\Discovery\DicoveryInterface::getDefinitions().
    */
   public function getDefinitions() {
+    $definitions = array();
     foreach (module_implements($this->hook) as $module) {
       $function = $module . '_' . $this->hook;
       foreach ($function() as $plugin_id => $definition) {