diff --git a/renderer.php b/renderer.php
index 7de8b646..1904bc73 100644
--- a/renderer.php
+++ b/renderer.php
@@ -575,7 +575,7 @@ public function correct_response(question_attempt $qa) {
             }
         }
 
-        $uclang = ucwords($currentlanguage);
+        $uclang = ucwords($currentlanguage ?? '');  // Current language can be null when prototype missing.
         $heading = get_string('asolutionis', 'qtype_coderunner');
         $heading = substr($heading, 0, strlen($heading) - 1) . ' (' . $uclang . ')';
         $divid = 'id_div_' . $fieldname;