diff --git a/index.js b/index.js
index 09522ca..7ac4e0e 100644
--- a/index.js
+++ b/index.js
@@ -18,7 +18,8 @@ var optionsBuilder = function () {
 
   return {
     withMessage: function(m) {
-      message = m;
+      m = m || ''
+      message = typeof m !== 'string'?m.toString():m;
       return this;
     },