diff --git a/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php b/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php
index 2088ae1..7bf4bfc 100644
--- a/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php
@@ -69,9 +69,8 @@ public function getDefinitions() {
     $reader->addNamespace('Drupal\Core\Annotation', array(DRUPAL_ROOT . '/core/lib/Drupal/Core/Annotation'));
 
     // Add the namespaces from the main plugin annotation, like @EntityType.
-    $explode = explode('\\', $this->pluginDefinitionAnnotationName);
-    array_pop($explode);
-    $reader->addNamespace(implode('\\', $explode));
+    $namespace = substr($this->pluginDefinitionAnnotationName, 0, strrpos($this->pluginDefinitionAnnotationName, '\\'));
+    $reader->addNamespace($namespace);
 
     // Clear the annotation loaders of any previous annotation classes.
     AnnotationRegistry::reset();