diff --git a/test/collections.js b/test/collections.js
index 71ef5a71c..18b242769 100644
--- a/test/collections.js
+++ b/test/collections.js
@@ -275,6 +275,7 @@ $(document).ready(function() {
 
   test('collections: size', function() {
     equal(_.size({one : 1, two : 2, three : 3}), 3, 'can compute the size of an object');
+    equal(_.size([1, 2, 3]), 3, 'can compute the size of an array');
   });
 
 });
diff --git a/underscore.js b/underscore.js
index 21a45c0d9..c699c991b 100644
--- a/underscore.js
+++ b/underscore.js
@@ -309,7 +309,7 @@
 
   // Return the number of elements in an object.
   _.size = function(obj) {
-    return _.toArray(obj).length;
+    return _.isArray(obj) ? obj.length : _.keys(obj).length;
   };
 
   // Array Functions