diff --git a/lib/tiles/OfflineTilesEnablerLayer.js b/lib/tiles/OfflineTilesEnablerLayer.js
index af43db3..76e42bf 100644
--- a/lib/tiles/OfflineTilesEnablerLayer.js
+++ b/lib/tiles/OfflineTilesEnablerLayer.js
@@ -485,9 +485,9 @@ define([
          */
         estimateTileSize : function(callback)
         {
-            if(_lastTileUrl)
+            if(this._lastTileUrl)
             {
-                var url = this.offline.proxyPath? this.offline.proxyPath + "?" +  _lastTileUrl : _lastTileUrl;
+                var url = this.offline.proxyPath? this.offline.proxyPath + "?" +  this._lastTileUrl : this._lastTileUrl;
                 request.get(url,{
                     handleAs: "text/plain; charset=x-user-defined",
                     headers: {