diff --git a/iron-request.html b/iron-request.html
index bc66f75..dbbaa7a 100644
--- a/iron-request.html
+++ b/iron-request.html
@@ -420,6 +420,11 @@
      * @return {string} .
      */
     _wwwFormUrlEncodePiece: function(str) {
+      // Support undefined input 
+      if (typeof(str) == 'undefined' || !str) {
+          return '';
+      }
+      
       // Spec says to normalize newlines to \r\n and replace %20 spaces with +.
       // jQuery does this as well, so this is likely to be widely compatible.
       return encodeURIComponent(str.toString().replace(/\r?\n/g, '\r\n'))