Error using DateTimePicker: Microsoft JScript runtime error: Object required

[ted231231]

I'm trying to use DateTimePicker (asp.net 4.0, c#).When the page is loaded, it's displayed on the screen with the requested date but when I try to change date or do anything else, the "Object Required" exception/error is thrown and I'd like to know how to handle it. On the top of my page I put \[code\]<%@ Register TagPrefix="cc1" Namespace="mycompany.WebServerControls.DateTimePicker" Assembly="DateTimePicker" %>\[/code\]The DateTimePicker is declared like this:\[code\]<cc1:datetimepicker id="dtpStart" runat="server" Width="129px" Height="36px"></cc1:datetimepicker>\[/code\]What am I doing wrong?