diff -u b/core/lib/Drupal/Core/Field/WidgetBase.php b/core/lib/Drupal/Core/Field/WidgetBase.php
--- b/core/lib/Drupal/Core/Field/WidgetBase.php
+++ b/core/lib/Drupal/Core/Field/WidgetBase.php
@@ -365,18 +365,21 @@
     $parents = $parent_element['#field_parents'];
     $field_state = static::getWidgetState($parents, $field_name, $form_state);
     $user_input = $form_state->getUserInput();
-    $field_values = [];
-    foreach ($user_input[$field_name] as $key => $input) {
-      if (is_numeric($key) && $key >= $delta) {
-        if ((int) $key === $delta) {
-          continue;
+    $field_input = NestedArray::getValue($user_input, $parent_element['#parents'], $exists);
+    if ($exists) {
+      $field_values = [];
+      foreach ($field_input as $key => $input) {
+        if (is_numeric($key) && $key >= $delta) {
+          if ((int) $key === $delta) {
+            continue;
+          }
+          --$key;
         }
-        --$key;
+        $field_values[$key] = $input;
       }
-      $field_values[$key] = $input;
+      NestedArray::setValue($user_input, $parent_element['#parents'], $field_values);
+      $form_state->setUserInput($user_input);
     }
-    $user_input[$field_name] = $field_values;
-    $form_state->setUserInput($user_input);
 
     unset($parent_element[$delta]);
     NestedArray::setValue($form, $array_parents, $parent_element);