diff --git a/lib/util.js b/lib/util.js
index 35bd93d6..b4486ef4 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -247,8 +247,9 @@ exports.isAbsolute = function(aPath) {
  * @param aRoot The root path or URL.
  * @param aPath The path or URL to be made relative to aRoot.
  */
-function relative(aRoot, aPath) {
-  if (aRoot === "") {
+function relative(aRoot, aPath) {  
+
+  if (typeof aRoot == 'undefined' || aRoot === "") {
     aRoot = ".";
   }