diff --git a/src/zoomifytilesource.js b/src/zoomifytilesource.js
index 3cf48071..7b62a6a8 100644
--- a/src/zoomifytilesource.js
+++ b/src/zoomifytilesource.js
@@ -44,7 +44,7 @@
      * @param {String} tilesUrl
      */
     $.ZoomifyTileSource = function(options) {
-        if(options.tileSize === undefined){
+        if(typeof options.tileSize === 'undefined'){
             options.tileSize = 256;
         }