I am attaching a sample web page based on the code that you pasted and on this on-line demo that worked properly at my side. Please review it and let me know if you have further questions.
Hope this will be helpful.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed