diff --git a/src/Plugin/search_api/processor/AddHierarchy.php b/src/Plugin/search_api/processor/AddHierarchy.php
index cb78e77..75d1cb0 100644
--- a/src/Plugin/search_api/processor/AddHierarchy.php
+++ b/src/Plugin/search_api/processor/AddHierarchy.php
@@ -7,8 +7,10 @@ use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Entity\Plugin\DataType\EntityAdapter;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Plugin\PluginFormInterface;
 use Drupal\search_api\Item\FieldInterface;
 use Drupal\search_api\Item\ItemInterface;
+use Drupal\search_api\Plugin\PluginFormTrait;
 use Drupal\search_api\Processor\ProcessorPluginBase;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
@@ -22,7 +24,9 @@ use Symfony\Component\DependencyInjection\ContainerInterface;
  *   }
  * )
  */
-class AddHierarchy extends ProcessorPluginBase {
+class AddHierarchy extends ProcessorPluginBase implements PluginFormInterface {
+
+  use PluginFormTrait;
 
   /**
    * The entity type manager.
@@ -79,8 +83,6 @@ class AddHierarchy extends ProcessorPluginBase {
    * {@inheritdoc}
    */
   public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
-    $form = parent::buildConfigurationForm($form, $form_state);
-
     $field_options = array();
     foreach ($this->index->getFields() as $field_id => $field) {
       if ($this->isEntityReferenceField($field)) {
@@ -103,8 +105,6 @@ class AddHierarchy extends ProcessorPluginBase {
    * {@inheritdoc}
    */
   public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
-    parent::validateConfigurationForm($form, $form_state);
-
     $fields = array_filter($form_state->getValues()['fields']);
     if ($fields) {
       $fields = array_keys($fields);
@@ -119,7 +119,7 @@ class AddHierarchy extends ProcessorPluginBase {
     /** @var \Drupal\search_api\Item\ItemInterface $item */
     foreach ($items as $item) {
       foreach ($item->getFields() as $field) {
-        if ($this->isEntityReferenceField($field) && in_array($field->getFieldIdentifier(), $this->configuration['fields'])) {
+        if (in_array($field->getFieldIdentifier(), $this->configuration['fields'])) {
           // The field is an entity reference, so we have to traverse up the
           // tree to find all parents.
           $hierarchyValues = array();