diff --git a/lib/OpenLayers/Popup/AnchoredBubble.js b/lib/OpenLayers/Popup/AnchoredBubble.js
index 0960f68b28..83cc46e84a 100644
--- a/lib/OpenLayers/Popup/AnchoredBubble.js
+++ b/lib/OpenLayers/Popup/AnchoredBubble.js
@@ -49,7 +49,7 @@ OpenLayers.Popup.AnchoredBubble.prototype =
         var contentSize = this.size.clone();
         contentSize.h -= (2 * OpenLayers.Popup.AnchoredBubble.CORNER_SIZE);
 
-        var id = this.div.id + "-contentDiv";
+        var id = this.div.id + "_contentDiv";
         this.contentDiv = OpenLayers.Util.createDiv(id, null, contentSize, 
                                                     null, "relative", null,
                                                     "hidden");