diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index d5a3617..b3ea83f 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -2219,6 +2219,15 @@ function system_update_8046() {
  * @ingroup config_upgrade
  */
 function system_update_8047() {
+  // Update default interface key change (from default-system to default) in value,
+  // before convert this variable to config.
+  $mail_system_settings= varible_get('mail_system');
+  $mail_system_settings['default'] = $mail_system_settings['default-system'];
+
+  // Reset default-system.
+  unset($mail_system_settings['default-system']);
+  variable_set('mail_system', $mail_system_settings);
+
   update_variables_to_config('system.mail', array(
     'mail_system' => 'interface',
   ));