.../Core/Config/Entity/DraggableListController.php | 47 ++++++++++------------
 1 file changed, 22 insertions(+), 25 deletions(-)

diff --git a/core/lib/Drupal/Core/Config/Entity/DraggableListController.php b/core/lib/Drupal/Core/Config/Entity/DraggableListController.php
index 5370be5..5f3dd13 100644
--- a/core/lib/Drupal/Core/Config/Entity/DraggableListController.php
+++ b/core/lib/Drupal/Core/Config/Entity/DraggableListController.php
@@ -14,6 +14,9 @@
 
 /**
  * Provides a list controller for draggable configuration entities.
+ *
+ * Using this class as a list controller requires the entity type to define a
+ * weight key.
  */
 abstract class DraggableListController extends ConfigEntityListController implements FormInterface {
 
@@ -44,21 +47,20 @@
   public function __construct($entity_type, array $entity_info, EntityStorageControllerInterface $storage, ModuleHandlerInterface $module_handler) {
     parent::__construct($entity_type, $entity_info, $storage, $module_handler);
 
-    // Check if the entity type supports weighting.
-    if (!empty($this->entityInfo['entity_keys']['weight'])) {
-      $this->weightKey = $this->entityInfo['entity_keys']['weight'];
+    // An entity type that does not support weighting should not use this list
+    // controller.
+    if (empty($this->entityInfo['entity_keys']['weight'])) {
+      throw new \Exception("Entity type '$entity_type' uses DraggableListController but does not define a weight key.");
     }
+
+    $this->weightKey = $this->entityInfo['entity_keys']['weight'];
   }
 
   /**
    * {@inheritdoc}
    */
   public function buildHeader() {
-    $header = array();
-    if (!empty($this->weightKey)) {
-      $header['weight'] = t('Weight');
-    }
-    return $header + parent::buildHeader();
+    return array('weight' => t('Weight')) + parent::buildHeader();
   }
 
   /**
@@ -66,19 +68,17 @@ public function buildHeader() {
    */
   public function buildRow(EntityInterface $entity) {
     $row = array();
-    if (!empty($this->weightKey)) {
-      // Override default values to markup elements.
-      $row['#attributes']['class'][] = 'draggable';
-      $row['#weight'] = $entity->get($this->weightKey);
-      // Add weight column.
-      $row['weight'] = array(
-        '#type' => 'weight',
-        '#title' => t('Weight for @title', array('@title' => $entity->label())),
-        '#title_display' => 'invisible',
-        '#default_value' => $entity->get($this->weightKey),
-        '#attributes' => array('class' => array('weight')),
-      );
-    }
+    // Override default values to markup elements.
+    $row['#attributes']['class'][] = 'draggable';
+    $row['#weight'] = $entity->get($this->weightKey);
+    // Add weight column.
+    $row['weight'] = array(
+      '#type' => 'weight',
+      '#title' => t('Weight for @title', array('@title' => $entity->label())),
+      '#title_display' => 'invisible',
+      '#default_value' => $entity->get($this->weightKey),
+      '#attributes' => array('class' => array('weight')),
+    );
     return $row + parent::buildRow($entity);
   }
 
@@ -86,10 +86,7 @@ public function buildRow(EntityInterface $entity) {
    * {@inheritdoc}
    */
   public function render() {
-    if (!empty($this->weightKey)) {
-      return drupal_get_form($this);
-    }
-    return parent::render();
+    return drupal_get_form($this);
   }
 
   /**