diff -u b/core/modules/image/image.module b/core/modules/image/image.module
--- b/core/modules/image/image.module
+++ b/core/modules/image/image.module
@@ -422,7 +422,7 @@
 
   $fid = $entity->settings['default_image']['fid'];
   if ($fid) {
-    $original_fid = isset($entity->original) ? $fid : NULL;
+    $original_fid = isset($entity->original) ? $entity->original->settings['default_image']['fid'] : NULL;
     if ($entity->settings['default_image']['fid'] != $original_fid) {
       $image = \Drupal::service('image.factory')->get(file_load($fid)->getFileUri());
       $entity->settings['default_image']['width'] = $image->getWidth();
only in patch2:
unchanged:
--- a/core/modules/image/lib/Drupal/image/Plugin/Field/FieldType/ImageItem.php
+++ b/core/modules/image/lib/Drupal/image/Plugin/Field/FieldType/ImageItem.php
@@ -335,6 +335,7 @@ protected function defaultImageForm(array &$element, array $settings) {
       '#description' => t('Image to be shown if no image is uploaded.'),
       '#default_value' => empty($settings['default_image']['fid']) ? array() : array($settings['default_image']['fid']),
       '#upload_location' => $settings['uri_scheme'] . '://default_images/',
+      '#element_validate' => array(array($this, 'validateDefaultImageForm')),
     );
     $element['default_image']['alt'] = array(
       '#type' => 'textfield',
@@ -361,6 +362,34 @@ protected function defaultImageForm(array &$element, array $settings) {
   }
 
   /**
+   * Validates the managed_file element for the default Image form.
+   *
+   * This function ensures the fid is a scalar value and not an array. It is
+   * assigned as a #element_validate callback in
+   * \Drupal\image\Plugin\Field\FieldType\ImageItem::defaultImageForm().
+   *
+   * @param array $element
+   *   The form element to process.
+   * @param array $form_state
+   *   The form state.
+   */
+  public function validateDefaultImageForm(array &$element, array &$form_state) {
+    // Call the original validator to ensure referencing an existing file is
+    // only allowed if there are existing references.
+    file_managed_file_validate($element, $form_state);
+
+    // Consolidate the array value of this field to a single FID as #extended
+    // for default image is not TRUE and this is a single value.
+    if (isset($element['fids']['#value'][0])) {
+      $value = $element['fids']['#value'][0];
+    }
+    else {
+      $value = 0;
+    }
+    \Drupal::formBuilder()->setValue($element, $value, $form_state);
+  }
+
+  /**
    * {@inheritdoc}
    */
   public function isDisplayed() {