diff --git a/core/lib/Drupal/Component/Plugin/DefaultPluginBag.php b/core/lib/Drupal/Component/Plugin/DefaultPluginBag.php
index 1e54c45..b5baaa7 100644
--- a/core/lib/Drupal/Component/Plugin/DefaultPluginBag.php
+++ b/core/lib/Drupal/Component/Plugin/DefaultPluginBag.php
@@ -65,7 +65,7 @@ public function __construct(PluginManagerInterface $manager, array $configuratio
    * {@inheritdoc}
    */
   protected function initializePlugin($instance_id) {
-    $configuration = $this->configurations[$instance_id];
+    $configuration = isset($this->configurations[$instance_id]) ? $this->configurations[$instance_id] : array();
     if (!isset($configuration[$this->pluginKey])) {
       throw new PluginException(String::format("Unknown plugin ID '@instance'.", array('@instance' => $instance_id)));
     }
diff --git a/core/tests/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php b/core/tests/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php
new file mode 100644
index 0000000..01c652f
--- /dev/null
+++ b/core/tests/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php
@@ -0,0 +1,218 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Component\Plugin\DefaultPluginBagTest.
+ */
+
+namespace Drupal\Tests\Component\Plugin;
+
+use Drupal\Component\Plugin\DefaultPluginBag;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests the default plugin bag.
+ *
+ * @see \Drupal\Component\Plugin\DefaultPluginBag
+ *
+ * @group Drupal
+ * @group Drupal_Plugin
+ */
+class DefaultPluginBagTest extends UnitTestCase {
+
+  /**
+   * The mocked plugin manager.
+   *
+   * @var \PHPUnit_Framework_MockObject_MockObject|\Drupal\Component\Plugin\PluginManagerInterface
+   */
+  protected $pluginManager;
+
+  /**
+   * The tested plugin bag.
+   *
+   * @var \PHPUnit_Framework_MockObject_MockObject|\Drupal\Component\Plugin\DefaultPluginBag
+   */
+  protected $defaultPluginBag;
+
+  /**
+   * Stores all setup plugin instances.
+   *
+   * @var array
+   */
+  protected $pluginInstances;
+
+  /**
+   * Contains the plugin configuration.
+   *
+   * @var array
+   */
+  protected $config = array(
+    'banana' => array('id' => 'banana', 'key' => 'value'),
+    'cherry' => array('id' => 'cherry', 'key' => 'value'),
+    'apple' => array('id' => 'apple', 'key' => 'value'),
+  );
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Default plugin bag',
+      'description' => 'Tests the default plugin bag.',
+      'group' => 'PHP Storage',
+    );
+  }
+
+  protected function setUp() {
+    $this->pluginManager = $this->getMock('Drupal\Component\Plugin\PluginManagerInterface');
+    $definitions = $this->getPluginDefinitions();
+    $this->pluginManager->expects($this->any())
+      ->method('getDefinitions')
+      ->will($this->returnValue($definitions));
+
+    $this->pluginInstances = array();
+    $map = array();
+    foreach ($definitions as $plugin_id => $definition) {
+      // Create a mock plugin instance.
+      $mock = $this->getMock('Drupal\Component\Plugin\PluginInspectionInterface');
+      $mock->expects($this->any())
+        ->method('getPluginId')
+        ->will($this->returnValue($plugin_id));
+      $this->pluginInstances[$plugin_id] = $mock;
+
+      $map[] = array($plugin_id, $this->config[$plugin_id], $this->pluginInstances[$plugin_id]);
+    }
+    $this->pluginManager->expects($this->any())
+      ->method('createInstance')
+      ->will($this->returnValueMap($map));
+
+    $this->defaultPluginBag = new DefaultPluginBag($this->pluginManager, $this->config);
+  }
+
+  /**
+   * Tests the has method.
+   *
+   * @see \Drupal\Component\Plugin\DefaultPluginBag::has()
+   */
+  public function testHas() {
+    $definitions = $this->getPluginDefinitions();
+
+    $this->assertFalse($this->defaultPluginBag->has($this->randomName()), 'Nonexistent plugin found.');
+
+    foreach (array_keys($definitions) as $plugin_id) {
+      $this->assertTrue($this->defaultPluginBag->has($plugin_id));
+    }
+  }
+
+  /**
+   * Tests the get method.
+   *
+   * @see \Drupal\Component\Plugin\DefaultPluginBag::get()
+   */
+  public function testGet() {
+    $apple = $this->pluginInstances['apple'];
+
+    $this->assertEquals($apple, $this->defaultPluginBag->get('apple'));
+  }
+
+  /**
+   * Tests the get method with an non existing plugin ID.
+   *
+   * @expectedException \Drupal\Component\Plugin\Exception\PluginException
+   */
+  public function testGetNotExistingPlugin() {
+    $this->defaultPluginBag->get('pear');
+  }
+
+  /**
+   * Returns some example plugin definitions.
+   *
+   * @return array
+   *   The example plugin definitions.
+   */
+  protected function getPluginDefinitions() {
+    $definitions = array(
+      'apple' => array(
+        'id' => 'apple',
+        'label' => 'Apple',
+        'color' => 'green',
+        'class' => 'Drupal\plugin_test\Plugin\plugin_test\fruit\Apple',
+        'provider' => 'plugin_test',
+      ),
+      'banana' => array(
+        'id' => 'banana',
+        'label' => 'Banana',
+        'color' => 'yellow',
+        'uses' => array(
+          'bread' => 'Banana bread',
+        ),
+        'class' => 'Drupal\plugin_test\Plugin\plugin_test\fruit\Banana',
+        'provider' => 'plugin_test',
+      ),
+      'cherry' => array(
+        'id' => 'cherry',
+        'label' => 'Cherry',
+        'color' => 'red',
+        'class' => 'Drupal\plugin_test\Plugin\plugin_test\fruit\Cherry',
+        'provider' => 'plugin_test',
+      ),
+    );
+    return $definitions;
+  }
+
+  /**
+   * Provides test data for testSortHelper.
+   *
+   * @return array
+   *   The test data.
+   */
+  public function providerTestSortHelper() {
+    return array(
+      array('apple', 'apple', 0),
+      array('apple', 'cherry', -1),
+      array('cherry', 'apple', 1),
+      array('cherry', 'banana', 1),
+    );
+  }
+
+  /**
+   * Tests the sort helper.
+   *
+   * @param string $plugin_id_1
+   *   The first plugin ID.
+   * @param string $plugin_id_2
+   *   The second plugin ID.
+   * @param int $expected
+   *   The expected result.
+   *
+   * @dataProvider providerTestSortHelper
+   */
+  public function testSortHelper($plugin_id_1, $plugin_id_2, $expected) {
+    if ($expected != 0) {
+      $expected = $expected > 0 ? 1 : -1;
+    }
+    $this->assertEquals($expected, $this->defaultPluginBag->sortHelper($plugin_id_1, $plugin_id_2));
+  }
+
+  /**
+   * Tests the configuration getter method.
+   *
+   * @see \Drupal\Component\Plugin\DefaultPluginBag::getConfiguration()
+   */
+  public function testGetConfiguration() {
+    // The expected order matches $this->config.
+    $expected = array('banana', 'cherry', 'apple');
+
+    $config = $this->defaultPluginBag->getConfiguration();
+    $this->assertSame($expected, array_keys($config), 'The order of the configuration is unchanged.');
+
+    $ids = $this->defaultPluginBag->getInstanceIDs();
+    $this->assertSame($expected, array_keys($ids), 'The order of the instances is unchanged.');
+
+    $this->defaultPluginBag->sort();
+    $config = $this->defaultPluginBag->getConfiguration();
+    $this->assertSame($expected, array_keys($config), 'After sorting, the order of the configuration is unchanged.');
+
+    $ids = $this->defaultPluginBag->getInstanceIDs();
+    sort($expected);
+    $this->assertSame($expected, array_keys($ids), 'After sorting, the order of the instances is also sorted.');
+  }
+
+}