diff --git a/src/Plugin/Field/FieldWidget/EntityReferenceDragDropWidget.php b/src/Plugin/Field/FieldWidget/EntityReferenceDragDropWidget.php
index 5ad6323..ac7331a 100644
--- a/src/Plugin/Field/FieldWidget/EntityReferenceDragDropWidget.php
+++ b/src/Plugin/Field/FieldWidget/EntityReferenceDragDropWidget.php
@@ -167,7 +167,9 @@ class EntityReferenceDragDropWidget extends OptionsWidgetBase implements Contain
     $element['target_id'] = array(
       '#type' => 'hidden',
       '#default_value' => implode(',', array_keys($selected)),
-      '#element_validate' => array('entityreference_dragdrop_element_validate'),
+      '#element_validate' => array(
+        array($this, 'entityreference_dragdrop_element_validate')
+      ),
       '#attached' => array(
         'library' => array('entityreference_dragdrop/init'),
         'drupalSettings' => array(
@@ -339,4 +341,11 @@ class EntityReferenceDragDropWidget extends OptionsWidgetBase implements Contain
     return $options;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function entityreference_dragdrop_element_validate($element, FormStateInterface $form_state) {
+    // TODO: Implement element_validate() function.
+  }
+
 }