I am adding web controls dynamically and everything works fine until I've
tried to add CalendarExtender from ASP.NET Ajax library.  The
CalanderExtender gets added and works fine until I submit the form and then
I get the following error:

Microsoft JScript runtime error:
Sys.WebForms.PageRequestManagerServerErrorException: Failed to load
viewstate.  The control tree into which viewstate is
being loaded must match the control tree that was used to save viewstate
during the previous request.  For example, when adding controls dynamically,
the controls added during a post-back must match the type and position of
the controls added during the initial request.

This error occurs before the Page_Load event.

Does anyone know how to fix this?

Here's how I am adding the Calendar Extender

txt = new TextBox();
txt.MaxLength = parm.MaxLength;
txt.ID = parm.ParameterNameID;
txt.Text = "";
txt.EnableViewState = true;
txt.AutoPostBack = false;
lbl.AssociatedControlID = txt.ID;
this.PlaceHolder1.Controls.Add(txt);

CalendarExtender ce = new CalendarExtender();
ce.ID = parm.ParameterNameID + "_CalendarExtender";
ce.TargetControlID = parm.ParameterNameID;
ce.Format = "d";
ce.EnableViewState = true;
this.PlaceHolder1.Controls.Add(ce);

Thank You

Peter