diff --git a/lib/OpenLayers/Handler/MouseWheel.js b/lib/OpenLayers/Handler/MouseWheel.js
index a64747e47e..135edb39a5 100644
--- a/lib/OpenLayers/Handler/MouseWheel.js
+++ b/lib/OpenLayers/Handler/MouseWheel.js
@@ -163,9 +163,6 @@ OpenLayers.Handler.MouseWheel = OpenLayers.Class(OpenLayers.Handler, {
         if (!overScrollableDiv && overMapDiv) {
             if (allowScroll) {
                 var delta = 0;
-                if (!e) {
-                    e = window.event;
-                }
                 
                 if (e.wheelDelta) {
                     delta = e.wheelDelta;
@@ -183,9 +180,11 @@ OpenLayers.Handler.MouseWheel = OpenLayers.Class(OpenLayers.Handler, {
 
                 if(this.interval) {
                     window.clearTimeout(this._timeoutId);
+                    // store e because window.event might change during delay
+                    var evt = OpenLayers.Util.extend({}, e);
                     this._timeoutId = window.setTimeout(
                         OpenLayers.Function.bind(function(){
-                            this.wheelZoom(e);
+                            this.wheelZoom(evt);
                         }, this),
                         this.interval
                     );