diff --git a/lib/tiles/offlineTilesEnabler.js b/lib/tiles/offlineTilesEnabler.js
index af561fc..c83ac50 100644
--- a/lib/tiles/offlineTilesEnabler.js
+++ b/lib/tiles/offlineTilesEnabler.js
@@ -91,6 +91,8 @@ define([
                         var img = query("img[src="+tileid+"]")[0];
                         var imgURL;
 
+                        console.assert(typeof img != "undefined","undefined image detected");
+
                         if( success )
                         {
                             img.style.borderColor = "blue";