diff --git a/lib/OpenLayers/Layer/FixedZoomLevels.js b/lib/OpenLayers/Layer/FixedZoomLevels.js
index 6d08cf4bd4..7907d34aea 100644
--- a/lib/OpenLayers/Layer/FixedZoomLevels.js
+++ b/lib/OpenLayers/Layer/FixedZoomLevels.js
@@ -77,12 +77,18 @@ OpenLayers.Layer.FixedZoomLevels.prototype = {
      * @type float
      */
     getResolution: function() {
+        var resolution = null;
+        
         var viewSize = this.map.getSize();
         var extent = this.getExtent();
-        return Math.max( extent.getWidth()  / viewSize.w,
-                         extent.getHeight() / viewSize.h );
-    },
-    
+        
+        if ((viewSize != null) && (extent != null)) {
+            resolution = Math.max( extent.getWidth()  / viewSize.w,
+                                   extent.getHeight() / viewSize.h );
+        }
+        return resolution;
+     },
+
     /** Calculates using px-> lonlat translation functions on tl and br 
      *   corners of viewport
      *