Closed Issue: Extender doesn't work for a control inside a WebPartZone [16407]

Have a div like the following:

<div>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<br />
<asp:WebPartManager ID="WebPartManager1" runat="server">
</asp:WebPartManager>
<br />
<asp:WebPartZone ID="WebPartZone1" runat="server" Height="150px" Width="304px">
<ZoneTemplate>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<cc1:CalendarExtender ID="TextBox1_CalendarExtender" runat="server" Enabled="True"
TargetControlID="TextBox1">
</cc1:CalendarExtender>
</ZoneTemplate>
</asp:WebPartZone>
</div>

View the page in browser.

RESULT:
Exception!

Exception Details: System.InvalidOperationException: The TargetControlID of 'TextBox1_CalendarExtender' is not valid. A control with ID 'TextBox1' could not be found.
Comments: JuergenGutsch, thank you for sharing your experience!