diff --git a/source/Ox.js b/source/Ox.js
index 184502de..6a17fff7 100644
--- a/source/Ox.js
+++ b/source/Ox.js
@@ -3866,7 +3866,7 @@ Ox.highlight <f> Highlight matches in a string
 @*/
 // fixme: with regexp, special chars would have to be escaped
 Ox.highlight = function(txt, str, classname) {
-    return str.length ? txt.replace(
+    return str && str.length ? txt.replace(
         new RegExp('(' + str + ')', 'ig'),
         '<span class="' + classname + '">$1</span>'
     ) : txt;