diff --git a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
index 140a1ad..9209783 100644
--- a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
+++ b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
@@ -223,8 +223,8 @@ protected function findDefinitions() {
       $this->moduleHandler->alter($this->alterHook, $definitions);
     }
     // If this plugin was provided by a Drupal module, ensure it is enabled.
-    foreach ($definitions as $plugin_id => $definition) {
-      if ($definition['provider'] && !in_array($definition['provider'], array('Core', 'Component')) && !$this->moduleHandler->moduleExists($definition['provider'])) {
+    foreach ($definitions as $plugin_id => $plugin_definition) {
+      if (isset($plugin_definition['provider']) && !in_array($plugin_definition['provider'], array('Core', 'Component')) && !$this->moduleHandler->moduleExists($plugin_definition['provider'])) {
         unset($definitions[$plugin_id]);
       }
     }
diff --git a/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerUnitTest.php b/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerUnitTest.php
index 45ad0d2..de471ef 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerUnitTest.php
@@ -7,10 +7,6 @@
 
 namespace Drupal\Tests\Core\Extension;
 
-if (!defined('DRUPAL_ROOT')) {
-  define('DRUPAL_ROOT', dirname(dirname(substr(__DIR__, 0, -strlen(__NAMESPACE__)))));
-}
-
 use Drupal\Core\Extension\ModuleHandler;
 use Drupal\Tests\UnitTestCase;
 
diff --git a/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php b/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
index bd6ba26..d1327ad 100644
--- a/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
@@ -77,6 +77,31 @@ public function testDefaultPluginManager() {
   }
 
   /**
+   * Tests the plugin manager with a disabled module.
+   */
+  public function testDefaultPluginManagerWithDisabledModule() {
+    $definitions = $this->expectedDefinitions;
+    $definitions['cherry'] = array(
+      'id' => 'cherry',
+      'label' => 'Cherry',
+      'color' => 'red',
+      'class' => 'Drupal\plugin_test\Plugin\plugin_test\fruit\Cherry',
+      'provider' => 'disabled_module',
+    );
+
+    $module_handler = $this->getMock('Drupal\Core\Extension\ModuleHandlerInterface');
+
+    $module_handler->expects($this->once())
+      ->method('moduleExists')
+      ->with('disabled_module')
+      ->will($this->returnValue(FALSE));
+
+    $plugin_manager = new TestPluginManager($this->namespaces, $definitions, $module_handler, 'test_alter_hook');
+
+    $this->assertEmpty($plugin_manager->getDefinition('cherry'), 'Plugin information of a disabled module still available');
+  }
+
+  /**
    * Tests the plugin manager with no cache and altering.
    */
   public function testDefaultPluginManagerWithAlter() {
@@ -157,3 +182,7 @@ public function testDefaultPluginManagerWithFilledCache() {
   }
 
 }
+
+if (!defined('DRUPAL_ROOT')) {
+  define('DRUPAL_ROOT', dirname(dirname(substr(__DIR__, 0, -strlen(__NAMESPACE__)))));
+}