could not be found for the trigger in UpdatePanel

could not be found for the trigger in UpdatePanel

I need to keep the button in usercontrol (.ascx) file. and i have given
the AsyncPostBackTrigger controlid for the same button. but its showing
"ERROR" like could not be found for the trigger in UpdatePanel. could any
one help me? this i have give controlid `
<Triggers>
<asp:AsyncPostBackTrigger ControlID="btn1" EventName="Click" />
</Triggers>
`
this button i have kept in user control
<asp:Button ID="btn1" runat="server" Text="Click" />