diff --git a/lib/utils.js b/lib/utils.js
index cf6aacc..ed08fc0 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -92,7 +92,7 @@ var utils = {
 
   defaults: function(object, defaults) {
     for (var key in defaults) {
-      if (object[key] === undefined) {
+      if (!(key in object)) {
         object[key] = defaults[key];
       }
     }