diff --git a/lib/OpenLayers/Layer/WMS/Untiled.js b/lib/OpenLayers/Layer/WMS/Untiled.js
index 0d56800406..732608a199 100644
--- a/lib/OpenLayers/Layer/WMS/Untiled.js
+++ b/lib/OpenLayers/Layer/WMS/Untiled.js
@@ -22,10 +22,8 @@ OpenLayers.Layer.WMS.Untiled.prototype =
                      },
 
     
-    /** WMS.Untiled layer is never a base layer. 
-     * @type Boolean
-     */
-    isBaseLayer: false,
+    /** @type Boolean */
+    isBaseLayer: null,
     
     /** @type DOMElement */
     imgDiv: null,
@@ -54,6 +52,11 @@ OpenLayers.Layer.WMS.Untiled.prototype =
                            OpenLayers.Util.upperCaseObject(this.DEFAULT_PARAMS)
                            );
         }
+
+        // if the layer is transparent, it will be an overlay        
+        this.isBaseLayer = ((this.params.TRANSPARENT != "true") && 
+                            (this.params.TRANSPARENT != true));
+
     },    
 
     /**