diff --git a/core/lib/Drupal/Core/Field/WidgetBase.php b/core/lib/Drupal/Core/Field/WidgetBase.php
index a5549e67..9c985bc9 100644
--- a/core/lib/Drupal/Core/Field/WidgetBase.php
+++ b/core/lib/Drupal/Core/Field/WidgetBase.php
@@ -388,7 +388,9 @@ public function extractFormValues(FieldItemListInterface $items, array $form, Fo
         // The original delta, before drag-and-drop reordering, is needed to
         // route errors to the correct form element.
         foreach ($values as $delta => &$value) {
-          $value['_original_delta'] = $delta;
+          if (is_array($value)) {
+            $value['_original_delta'] = $delta;
+          }
         }
 
         usort($values, function ($a, $b) {