diff --git a/test/arrays.js b/test/arrays.js
index 54cd9f8d2..ff430648d 100644
--- a/test/arrays.js
+++ b/test/arrays.js
@@ -89,6 +89,28 @@ $(document).ready(function() {
 
     var result = (function(){ return _.uniq(arguments); })(1, 2, 1, 3, 1, 4);
     equal(result.join(', '), '1, 2, 3, 4', 'works on an arguments object');
+
+    var list = [];
+    list[2] = list[3] = null;
+    list[8] = 2;
+    list[10] = 2;
+    list[11] = 5;
+    list[14] = 5;
+    list[16] = 8;
+    list[19] = 8;
+    list[26] = list[29] = undefined;
+    list[33] = "hi";
+
+    var result = _.uniq(list, true);
+    if (0 in [undefined]) {
+      // According to the JScript ES 3 spec, section 2.1.26, JScript 5.x (IE <=
+      // 8) treats `undefined` elements in arrays as elisions.
+      deepEqual(result, [null, 2, 5, 8, undefined, "hi"], "Works with sorted sparse arrays");
+      equal(result.length, 6, "The resulting array should not be sparse");
+    } else {
+      deepEqual(result, [null, 2, 5, 8, "hi"], "Works with sorted sparse arrays where `undefined` elements are elided");
+      equal(result.length, 5, "The resulting array should not be sparse");
+    }
   });
 
   test("arrays: intersection", function() {
diff --git a/underscore.js b/underscore.js
index a283e55ff..88da6e1d8 100644
--- a/underscore.js
+++ b/underscore.js
@@ -372,15 +372,17 @@
   // Aliased as `unique`.
   _.uniq = _.unique = function(array, isSorted, iterator) {
     var initial = iterator ? _.map(array, iterator) : array;
-    var result = [];
-    _.reduce(initial, function(memo, el, i) {
-      if (0 == i || (isSorted === true ? _.last(memo) != el : !_.include(memo, el))) {
-        memo[memo.length] = el;
-        result[result.length] = array[i];
+    var results = [];
+    // The `isSorted` flag is irrelevant if the array only contains two elements.
+    if (array.length < 3) isSorted = true;
+    _.reduce(initial, function (memo, value, index) {
+      if (isSorted ? _.last(memo) !== value || !memo.length : !_.include(memo, value)) {
+        memo.push(value);
+        results.push(array[index]);
       }
       return memo;
     }, []);
-    return result;
+    return results;
   };
 
   // Produce an array that contains the union: each distinct element from all of