Can not catch EnterKey Events in IE6

[gujun]

The source code as following can catch the EnterKey Events normally in IE8+, but it doesn't work in IE6:

<SpecialKey Fn="specialKeyFn" />

var specialKeyFn= function (field, e) {
    if (e.getKey() == Ext.EventObject.ENTER) {
        alert("Enter Key Pressed.");
    }
}

In fact the specialKeyFn function is not called at all.