diff --git a/test/collections.js b/test/collections.js
index 18b242769..fb457f605 100644
--- a/test/collections.js
+++ b/test/collections.js
@@ -235,6 +235,9 @@ $(document).ready(function() {
     var people = [{name : 'curly', age : 50}, {name : 'moe', age : 30}];
     people = _.sortBy(people, function(person){ return person.age; });
     equal(_.pluck(people, 'name').join(', '), 'moe, curly', 'stooges sorted by age');
+
+    var list = [undefined, 4, 1, undefined, 3, 2];
+    equal(_.sortBy(list, _.identity).join(','), '1,2,3,4,,', 'sortBy with undefined values');
   });
 
   test('collections: groupBy', function() {
diff --git a/underscore.js b/underscore.js
index 00ca13d4a..e3db3646d 100644
--- a/underscore.js
+++ b/underscore.js
@@ -270,8 +270,8 @@
       };
     }).sort(function(left, right) {
       var a = left.criteria, b = right.criteria;
-      if (_.isUndefined(a)) return 1;
-      if (_.isUndefined(b)) return -1;
+      if (a === void 0) return 1;
+      if (b === void 0) return -1;
       return a < b ? -1 : a > b ? 1 : 0;
     }), 'value');
   };