diff --git a/core/lib/Drupal/Component/Plugin/PluginBag.php b/core/lib/Drupal/Component/Plugin/PluginBag.php
index e799d2d..4aef593 100644
--- a/core/lib/Drupal/Component/Plugin/PluginBag.php
+++ b/core/lib/Drupal/Component/Plugin/PluginBag.php
@@ -161,7 +161,11 @@ public function key() {
    */
   public function valid() {
     $key = key($this->instanceIDs);
-    return ($key !== NULL) && ($key !== FALSE) && $this->get($key);
+    // Check the key is valid but also that this key yields a plugin from get().
+    // There can be situations where configuration contains data for a plugin
+    // that cannot be instantiated. In this case, this enables us to skip that
+    // plugin during iteration.
+    return $key !== NULL && $key !== FALSE && $this->get($key);
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php b/core/tests/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php
index f12c146..ae92285 100644
--- a/core/tests/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php
+++ b/core/tests/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php
@@ -144,7 +144,7 @@ public function testRemoveInstanceId() {
    * @see \Drupal\Component\Plugin\DefaultPluginBag::setConfiguration()
    */
   public function testSetConfiguration() {
-    $this->setupPluginBag($this->exactly(4));
+    $this->setupPluginBag($this->exactly(3));
     $expected = array(
       'id' => 'cherry',
       'key' => 'value',
diff --git a/core/tests/Drupal/Tests/Component/Plugin/PluginBagTestBase.php b/core/tests/Drupal/Tests/Component/Plugin/PluginBagTestBase.php
index c9e543b..461321d 100644
--- a/core/tests/Drupal/Tests/Component/Plugin/PluginBagTestBase.php
+++ b/core/tests/Drupal/Tests/Component/Plugin/PluginBagTestBase.php
@@ -75,12 +75,27 @@ protected function setupPluginBag(\PHPUnit_Framework_MockObject_Matcher_InvokedR
     $create_count = $create_count ?: $this->never();
     $this->pluginManager->expects($create_count)
       ->method('createInstance')
-      ->will($this->returnValueMap($map));
+      ->will($this->returnCallback(array($this, 'returnPluginMap')));
 
     $this->defaultPluginBag = new DefaultPluginBag($this->pluginManager, $this->config);
   }
 
   /**
+   * Return callback for createInstance.
+   *
+   * @param string $plugin_id
+   *   The plugin ID to return the mock plugin for.
+   *
+   * @return \Drupal\Component\Plugin\PluginInspectionInterface|\PHPUnit_Framework_MockObject_MockObject
+   *   The mock plugin object.
+   */
+  public function returnPluginMap($plugin_id) {
+    if (isset($this->pluginInstances[$plugin_id])) {
+      return $this->pluginInstances[$plugin_id];
+    }
+  }
+
+  /**
    * Returns a mocked plugin object.
    *
    * @param string $plugin_id