diff --git a/backbone.js b/backbone.js
index c793c652..469c479a 100644
--- a/backbone.js
+++ b/backbone.js
@@ -342,7 +342,7 @@
 
     // A model is new if it has never been saved to the server, and lacks an id.
     isNew : function() {
-      return this[this.idAttribute] == null;
+      return this.id == null;
     },
 
     // Call this method to manually fire a `change` event for this model.
diff --git a/test/model.js b/test/model.js
index 3bb8ffe4..7ad0d71f 100644
--- a/test/model.js
+++ b/test/model.js
@@ -175,8 +175,10 @@ $(document).ready(function() {
     var model = new MongoModel({id: 'eye-dee', _id: 25, title: 'Model'});
     equals(model.get('id'), 'eye-dee');
     equals(model.id, 25);
+    equals(model.isNew(), false);
     model.unset('_id');
     equals(model.id, undefined);
+    equals(model.isNew(), true);
   });
 
   test("Model: set an empty string", function() {