diff --git a/overviewer_core/data/js_src/util.js b/overviewer_core/data/js_src/util.js
index 4029c89..3ffdb84 100644
--- a/overviewer_core/data/js_src/util.js
+++ b/overviewer_core/data/js_src/util.js
@@ -159,7 +159,7 @@ overviewer.util = {
                 noWrap: true,
                 maxZoom: obj.maxZoom,
                 minZoom: obj.minZoom,
-                errorTileUrl: obj.base + obj.path + "/blank.png",
+                errorTileUrl: obj.base + obj.path + "/blank." + obj.imgextension,
             });
             myLayer.getTileUrl = overviewer.util.getTileUrlGenerator(obj.path, obj.base, obj.imgextension);