diff --git a/core/lib/Drupal/Core/Entity/EntityViewBuilder.php b/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
index 41beeaf..6d3b22d 100644
--- a/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
+++ b/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
@@ -266,10 +266,12 @@ public function resetCache(array $entities = NULL) {
    *   TRUE if the view mode can be cached, FALSE otherwise.
    */
   protected function isViewModeCacheable($view_mode) {
-    // The isset() checks below are necessary because 'default' is not an actual
-    // view mode.
+    if ($view_mode == 'default') {
+      // The 'default' is not an actual view mode.
+      return TRUE;
+    }
     $view_modes_info = entity_get_view_modes($this->entityType);
-    $view_mode_is_cacheable = !isset($view_modes_info[$view_mode]) || (isset($view_modes_info[$view_mode]) && $view_modes_info[$view_mode]['cache']);
-    return $view_mode_is_cacheable;
+    return !empty($view_modes_info[$view_mode]['cache']);
   }
+
 }
diff --git a/core/modules/entity/lib/Drupal/entity/Form/EntityDisplayModeFormBase.php b/core/modules/entity/lib/Drupal/entity/Form/EntityDisplayModeFormBase.php
index dc96922..39dff2d 100644
--- a/core/modules/entity/lib/Drupal/entity/Form/EntityDisplayModeFormBase.php
+++ b/core/modules/entity/lib/Drupal/entity/Form/EntityDisplayModeFormBase.php
@@ -109,6 +109,10 @@ public function form(array $form, array &$form_state) {
    *   TRUE if the display mode exists, FALSE otherwise.
    */
   public function exists($entity_id, array $element, array $form_state) {
+    // Do not allow to add internal 'default' view mode.
+    if ($entity_id == 'default') {
+      return TRUE;
+    }
     return (bool) $this->queryFactory
       ->get($this->entity->entityType())
       ->condition('id', $element['#field_prefix'] . $entity_id)