diff --git a/htdocs/account/index.php b/htdocs/account/index.php
index d22e3dc3e3d68b91f4fdac85c019b95090790215..180c566da45bf58691ef811a3d58ae4150f99b91 100644
--- a/htdocs/account/index.php
+++ b/htdocs/account/index.php
@@ -239,7 +239,7 @@ function accountprefs_submit(Pieform $form, $values) {
         }
     }
 
-    if ($values['theme'] == 'sitedefault') {
+    if (isset($values['theme']) && $values['theme'] == 'sitedefault') {
         $USER->set_account_preference('theme', '');
     }