diff --git a/js/src/builtin/Sorting.js b/js/src/builtin/Sorting.js
index d0b3483c6d2a..0451ceb7a2fb 100644
--- a/js/src/builtin/Sorting.js
+++ b/js/src/builtin/Sorting.js
@@ -230,11 +230,6 @@ function MoveHoles(sparse, sparseLen, dense, denseLen) {
 
 // Iterative, bottom up, mergesort.
 function MergeSort(array, len, comparefn) {
-    // To save effort we will do all of our work on a dense list,
-    // then create holes at the end.
-    var denseList = new List();
-    var denseLen = 0;
-
     // Until recently typed arrays had no sort method. To work around that
     // many users passed them to Array.prototype.sort. Now that we have a
     // typed array specific sorting method it makes sense to divert to it
@@ -243,6 +238,11 @@ function MergeSort(array, len, comparefn) {
         return callFunction(TypedArraySort, array, comparefn);
     }
 
+    // To save effort we will do all of our work on a dense list,
+    // then create holes at the end.
+    var denseList = new List();
+    var denseLen = 0;
+
     for (var i = 0; i < len; i++) {
         if (i in array)
             denseList[denseLen++] = array[i];
@@ -253,7 +253,7 @@ function MergeSort(array, len, comparefn) {
 
     // Insertion sort for small arrays, where "small" is defined by performance
     // testing.
-    if (len < 24) {
+    if (denseLen < 24) {
         InsertionSort(denseList, 0, denseLen - 1, comparefn);
         MoveHoles(array, len, denseList, denseLen);
         return array;