update panel not working in asp.net

Question

Am using the update panel in my website and try to create the AsyncPostBackTrigger for a listbox. But it show the below error.So i try to create this event in page_Init section. It didn't through any error message.But i try to click the last item in list box means it postback the control and go to the first item.Please help me to fix this..

A control with ID 'ddl_discount' could not be found for the trigger in UpdatePanel 'update_pan'.

 protected void Page_Init()
        {
            AsyncPostBackTrigger trigger = new AsyncPostBackTrigger();
            trigger.EventName = "SelectedIndexChanged";
            trigger.ControlID = ddl_discount.UniqueID.ToString();
            update_pan.Triggers.Add(trigger);
        }

Answer 1

Try to put script manager in page instead of master page.

No need for any trigger. It works for me.