diff --git a/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php b/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php
index 283c9f6..a33dbc1 100644
--- a/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php
@@ -47,6 +47,9 @@ class AnnotatedClassDiscovery implements DiscoveryInterface {
    *   Defaults to 'Drupal\Component\Annotation\Plugin'.
    */
   function __construct($plugin_namespaces = array(), $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
+    if (!is_string($plugin_definition_annotation_name)) {
+      throw new \InvalidArgumentException("Argument 1 must be a string.");
+    }
     $this->pluginNamespaces = $plugin_namespaces;
     $this->pluginDefinitionAnnotationName = $plugin_definition_annotation_name;
   }
diff --git a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
index d6b0609..9433b3d 100644
--- a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
+++ b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
@@ -101,6 +101,9 @@ class DefaultPluginManager extends PluginManagerBase implements PluginManagerInt
    *   Defaults to 'Drupal\Component\Annotation\Plugin'.
    */
   public function __construct($subdir, \Traversable $namespaces, $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
+    if (!is_string($plugin_definition_annotation_name)) {
+      throw new \InvalidArgumentException("Argument 2 must be a string.");
+    }
     $this->subdir = $subdir;
     $this->discovery = new AnnotatedClassDiscovery($subdir, $namespaces, $plugin_definition_annotation_name);
     $this->discovery = new ContainerDerivativeDiscoveryDecorator($this->discovery);
diff --git a/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
index d6b5378..c599214 100644
--- a/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
@@ -46,6 +46,9 @@ class AnnotatedClassDiscovery extends ComponentAnnotatedClassDiscovery {
    *   Defaults to 'Drupal\Component\Annotation\Plugin'.
    */
   function __construct($subdir, \Traversable $root_namespaces, $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
+    if (!is_string($plugin_definition_annotation_name)) {
+      throw new \InvalidArgumentException("Argument 2 must be a string.");
+    }
     if ($subdir) {
       $this->subdir = str_replace('/', '\\', $subdir);
     }
diff --git a/core/modules/search/lib/Drupal/search/SearchPluginManager.php b/core/modules/search/lib/Drupal/search/SearchPluginManager.php
index 3821791..ce0e17c 100644
--- a/core/modules/search/lib/Drupal/search/SearchPluginManager.php
+++ b/core/modules/search/lib/Drupal/search/SearchPluginManager.php
@@ -28,8 +28,7 @@ class SearchPluginManager extends DefaultPluginManager {
    * {@inheritdoc}
    */
   public function __construct(\Traversable $namespaces, ConfigFactory $config_factory) {
-    $annotation_namespaces = array('Drupal\search\Annotation' => $namespaces['Drupal\search']);
-    parent::__construct('Plugin/Search', $namespaces, $annotation_namespaces, 'Drupal\search\Annotation\SearchPlugin');
+    parent::__construct('Plugin/Search', $namespaces, 'Drupal\search\Annotation\SearchPlugin');
 
     $this->configFactory = $config_factory;
   }