diff --git a/lib/OpenLayers/Layer/WMS.js b/lib/OpenLayers/Layer/WMS.js
index 0bf43022ae..cfd7f173c9 100644
--- a/lib/OpenLayers/Layer/WMS.js
+++ b/lib/OpenLayers/Layer/WMS.js
@@ -48,7 +48,7 @@ OpenLayers.Layer.WMS.prototype =
                            );
         }
         
-        this.isBaseLayer = (this.params.TRANSPARENT != "true");
+        this.isBaseLayer = (this.params.TRANSPARENT != "true" && this.params.TRANSPARENT != true);
     },    
 
     /**