diff --git a/lib/OpenLayers/Layer/Grid.js b/lib/OpenLayers/Layer/Grid.js
index 9c3f70c0d4..f5837b15fc 100644
--- a/lib/OpenLayers/Layer/Grid.js
+++ b/lib/OpenLayers/Layer/Grid.js
@@ -660,7 +660,7 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
     getServerZoom: function() {
         var serverResolution = this.getServerResolution();
         return this.serverResolutions ?
-            this.serverResolutions.indexOf(serverResolution) :
+            OpenLayers.Util.indexOf(this.serverResolutions, serverResolution) :
             this.map.getZoomForResolution(serverResolution);
     },