diff --git a/lib/OpenLayers/Layer.js b/lib/OpenLayers/Layer.js
index c4bba40bad..ad79a36d57 100644
--- a/lib/OpenLayers/Layer.js
+++ b/lib/OpenLayers/Layer.js
@@ -409,15 +409,19 @@ OpenLayers.Layer.prototype = {
      * @type OpenLayers.LonLat
      */
     getLonLatFromViewPortPx: function (viewPortPx) {
-        var size = this.map.getSize();
-        var center = this.map.getCenter();
-        var res  = this.map.getResolution();
-    
-        var delta_x = viewPortPx.x - (size.w / 2);
-        var delta_y = viewPortPx.y - (size.h / 2);
+        var lonlat = null;
+        if (viewPortPx != null) {
+            var size = this.map.getSize();
+            var center = this.map.getCenter();
+            var res  = this.map.getResolution();
         
-        return new OpenLayers.LonLat(center.lon + delta_x * res ,
-                                     center.lat - delta_y * res); 
+            var delta_x = viewPortPx.x - (size.w / 2);
+            var delta_y = viewPortPx.y - (size.h / 2);
+            
+            lonlat = new OpenLayers.LonLat(center.lon + delta_x * res ,
+                                         center.lat - delta_y * res); 
+        }
+        return lonlat;
     },
 
     /**
@@ -428,12 +432,16 @@ OpenLayers.Layer.prototype = {
      * @type OpenLayers.Pixel
      */
     getViewPortPxFromLonLat: function (lonlat) {
-        var resolution = this.map.getResolution();
-        var extent = this.map.getExtent();
-        return new OpenLayers.Pixel(
-                       Math.round(1/resolution * (lonlat.lon - extent.left)),
-                       Math.round(1/resolution * (extent.top - lonlat.lat))
-                       );    
+        var px = null; 
+        if (lonlat != null) {
+            var resolution = this.map.getResolution();
+            var extent = this.map.getExtent();
+            px = new OpenLayers.Pixel(
+                           Math.round(1/resolution * (lonlat.lon - extent.left)),
+                           Math.round(1/resolution * (extent.top - lonlat.lat))
+                           );    
+        }
+        return px;
     },
     
     /**