diff --git a/lib/weblib.php b/lib/weblib.php
index eaf6cbba0a3..e93b369e321 100644
--- a/lib/weblib.php
+++ b/lib/weblib.php
@@ -2275,7 +2275,7 @@ function get_html_lang($dir = false) {
     global $CFG;
 
     $currentlang = current_language();
-    if ($currentlang !== $CFG->lang && !get_string_manager()->translation_exists($currentlang)) {
+    if (isset($CFG->lang) && $currentlang !== $CFG->lang && !get_string_manager()->translation_exists($currentlang)) {
         // Use the default site language when the current language is not available.
         $currentlang = $CFG->lang;
         // Fix the current language.