diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeCategoryForm.php b/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeCategoryForm.php
index dc50c87..243acac 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeCategoryForm.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeCategoryForm.php
@@ -24,7 +24,7 @@ public function getFormID() {
    */
   public function buildForm(array $form, array &$form_state, $cid = NULL) {
     $items = $this->aggregatorItemStorage->loadByCategory($cid);
-    return $this->prepareFormItems($items);
+    return parent::buildForm($form, $form_state, $items);
   }
 
 }
diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeFeedForm.php b/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeFeedForm.php
index c8d1f2a..ce4a04f 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeFeedForm.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeFeedForm.php
@@ -25,8 +25,9 @@ public function getFormID() {
    * {@inheritdoc}
    */
   public function buildForm(array $form, array &$form_state, FeedInterface $aggregator_feed = NULL) {
+    $this->feed = $aggregator_feed;
     $items = $this->aggregatorItemStorage->loadByFeed($aggregator_feed->id());
-    return $this->prepareFormItems($items, $aggregator_feed);
+    return parent::buildForm($form, $form_state, $items);
   }
 
 }
diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeFormBase.php b/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeFormBase.php
index 564adf7..3bae0ff 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeFormBase.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Form/CategorizeFormBase.php
@@ -8,7 +8,6 @@
 namespace Drupal\aggregator\Form;
 
 use Drupal\aggregator\FeedInterface;
-use Drupal\aggregator\ItemInterface;
 use Drupal\aggregator\ItemStorageControllerInterface;
 use Drupal\Component\Utility\String;
 use Drupal\Core\Config\Config;
@@ -51,6 +50,13 @@
   protected $aggregatorItemStorage;
 
   /**
+   * The feed to use.
+   *
+   * @var \Drupal\aggregator\FeedInterface
+   */
+  protected $feed;
+
+  /**
    * Constructs a \Drupal\aggregator\Controller\AggregatorController object.
    *
    * @param \Drupal\Core\Entity\EntityRenderControllerInterface $aggregator_item_renderer
@@ -82,20 +88,12 @@ public static function create(ContainerInterface $container) {
   }
 
   /**
-   * Prepares a list of items to be displayed in a form.
-   *
-   * @param \Drupal\aggregator\ItemInterface[] $items
-   *   An array of items.
-   * @param \Drupal\aggregator\FeedInterface|null $feed
-   *   (optional) The feed we are displaying.
-   *
-   * @return array
-   *   A form builder array.
+   * {@inheritdoc}
    */
-  protected function prepareFormItems(array $items, FeedInterface $feed = NULL) {
+  public function buildForm(array $form, array &$form_state, array $items = NULL) {
 
     $form['feed_source'] = array(
-      '#value' => $feed,
+      '#value' => $this->feed,
     );
     $categories = array();
     $done = FALSE;