diff --git b/core/modules/entity_ui/entity_ui.module a/core/modules/entity_ui/entity_ui.module
index dc09293..a2f6b08 100644
--- b/core/modules/entity_ui/entity_ui.module
+++ a/core/modules/entity_ui/entity_ui.module
@@ -107,9 +107,15 @@ function view_mode_load($machine_name) {
  *   The machine name as it was typed in a form element of type 'machine_name'.
  *
  * @param array $element
- *   The form element of type 'machine_name' that is being checked.
+ *   The form element of type 'machine_name' that is being checked. The element
+ *   is assumed to have an '#entity_type' key which denotes the entity type to
+ *   check for an existing view mode.
+ *
+ * @return bool
+ *   TRUE if a view mode with the given machine name already exists for the
+ *   entity type specified in the passed-in element; FALSE otherwise.
  */
 function entity_ui_view_mode_exists($typed_machine_name, $element) {
-  return entity_load('view_mode', $element['#entity_type'] . ".$typed_machine_name");
+  return (bool) entity_load('view_mode', $element['#entity_type'] . ".$typed_machine_name");
 }
 
diff --git b/core/modules/system/system.install a/core/modules/system/system.install
index ca9faa4..8f80ff2 100644
--- b/core/modules/system/system.install
+++ a/core/modules/system/system.install
@@ -2213,10 +2213,12 @@ function system_update_8034() {
  *
  * @ingroup config_upgrade
  */
-function system_update_8034() {
-  // We can't call entity_get_info() in an update hook, so we hardcode them.
-  // We only care about the core view modes as hook_entity_info_alter() will
-  // still work since that was the only way to add custom view modes in D7.
+function system_update_8035() {
+  // We cannot call entity_get_info() in an update hook, so we hardcode the view
+  // modes. Only the node entity type's teaser view mode is set to custom by
+  // default, we check specifically for that below. The only way to add custom
+  // view modes in Drupal 7 was hook_entity_info_alter(), which still works in
+  // Drupal 8.
   $entity_view_modes = array(
     'node' => array(
       'full' => 'Full content',