diff --git a/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php b/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php
index 667f365..20007d1 100644
--- a/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php
@@ -17,6 +17,11 @@
   /**
    * {@inheritdoc}
    */
+  abstract public function getDefinitions();
+
+  /**
+   * {@inheritdoc}
+   */
   public function getDefinition($plugin_id, $exception_on_invalid = TRUE) {
     $definitions = $this->getDefinitions();
     return $this->doGetDefinition($definitions, $plugin_id, $exception_on_invalid);
diff --git a/core/lib/Drupal/Component/Plugin/Discovery/StaticDiscovery.php b/core/lib/Drupal/Component/Plugin/Discovery/StaticDiscovery.php
index 72a7243..18f51b7 100644
--- a/core/lib/Drupal/Component/Plugin/Discovery/StaticDiscovery.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/StaticDiscovery.php
@@ -19,6 +19,9 @@ class StaticDiscovery implements DiscoveryInterface {
    * Implements Drupal\Component\Plugin\Discovery\DiscoveryInterface::getDefinitions().
    */
   public function getDefinitions() {
+    if (!$this->definitions) {
+      $this->definitions = array();
+    }
     return $this->definitions;
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php b/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
index bd1efc2..f9af6b2 100644
--- a/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
@@ -89,7 +89,7 @@ public function testDefaultPluginManagerWithDisabledModule() {
 
     $plugin_manager = new TestPluginManager($this->namespaces, $definitions, $module_handler, 'test_alter_hook');
 
-    $this->assertEmpty($plugin_manager->getDefinition('cherry'), 'Plugin information of a disabled module is not available');
+    $this->assertEmpty($plugin_manager->getDefinition('cherry', FALSE), 'Plugin information of a disabled module is not available');
   }
 
   /**