diff --git a/src/iiiftilesource.js b/src/iiiftilesource.js
index 1433a968..fd675753 100644
--- a/src/iiiftilesource.js
+++ b/src/iiiftilesource.js
@@ -145,7 +145,9 @@ $.IIIFTileSource = function( options ){
     if( this.sizes ) {
         var sizeLength = this.sizes.length;
         if ( (sizeLength === options.maxLevel) || (sizeLength === options.maxLevel + 1) ) {
-            this.levelSizes = this.sizes.slice().sort(( size1, size2 ) => size1.width - size2.width);
+            this.levelSizes = this.sizes.slice().sort(function( size1, size2 ) {
+                return size1.width - size2.width;
+            });
             // Need to take into account that the list may or may not include the full resolution size
             if( sizeLength === options.maxLevel ) {
                 this.levelSizes.push( {width: this.width, height: this.height} );