RadDropDownList problem with proccesskeypreview

[David]

I used following code to change the tab order by pressing Enter key , and it works great on any objects of my form.But when I changed the AutoCompleteMode property of my RadDropDownList to "SuggestAppend" , then
when I press Enter on RaddropDown the tab order not changed and it stay on this control !!!! plz help to handle this problem.

 protected override bool ProcessKeyPreview(ref Message m)
        {
            if (m.Msg == 0x0100 && (int)m.WParam == 13)
            {
                this.ProcessTabKey(true);
            }