If there's no options passed, this would fail. So let's check that

git-svn-id: http://svn.openlayers.org/trunk/openlayers@819 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/WFS.js

@@ -42,7 +42,7 @@ OpenLayers.Layer.WFS.prototype =
         OpenLayers.Layer.Grid.prototype.initialize.apply(this, newArguments);
         OpenLayers.Layer.Markers.prototype.initialize.apply(this, newArguments);
 
-        if (options['tileSize']) {
+        if (options && options['tileSize']) {
             this.tileSize = options['tileSize'].copyOf();
         } else {
             this.tileSize = new OpenLayers.Size(256, 256);