can not access control inside asp.net updatepanel

[LT Mote]

I use this official sample from Microsoft:When I try to access the MyLabel from codebehind in the click event the MyLabel is NULL.Why this?\[code\]<asp:ScriptManager ID="PageScriptManager" runat="server" /><asp:Button ID="MyButton" Text="Click Me" runat="server" onClick="MyButton_Clicked" /><asp:UpdatePanel ID="MyUpdatePanel" runat="server"><Triggers> <asp:AsyncPostBackTrigger ControlID="MyButton" EventName="Click" /></Triggers> <ContentTemplate> <asp:Label ID="MyLabel" Text="OldText" runat="server" /> </ContentTemplate></asp:UpdatePanel>private void MyButton_Clicked(object sender, EventArgs e) { MyLabel.Text = "NewText!"; } \[/code\]