diff --git a/mod/assign/externallib.php b/mod/assign/externallib.php
index 783c45507dcf2..a84149cee3f91 100644
--- a/mod/assign/externallib.php
+++ b/mod/assign/externallib.php
@@ -62,7 +62,7 @@ private static function generate_warning($assignmentid, $warningcode, $detail) {
             $message = 'Unknown warning type.';
         }
 
-        return array('item'=>$detail,
+        return array('item' => s($detail),
                      'itemid'=>$assignmentid,
                      'warningcode'=>$warningcode,
                      'message'=>$message);