A control with ID txtamt could not be found for the trigger in UpdatePanel UpTotAmt.

Here txtamt is the item template of a gridview and uptotamt is outside of the gridview.

<asp:UpdatePanel ID="UpTotAmt" runat="server" RenderMode="Inline" UpdateMode="Conditional">
                                <ContentTemplate>
                                     <asp:TextBox ID="txttot" runat="server" CssClass="texttot" Width="180px" 
                                    Enabled="False" ></asp:TextBox>
                                </ContentTemplate>
                                <Triggers >
                                <asp:AsyncPostBackTrigger ControlID="txtamt" EventName="TextChanged" />
                                </Triggers>
                            </asp:UpdatePanel>