From the provided information it would be hard to point the reason for the error you are getting.
Could you elaborate more on the scenario you have? Could you also share a runnable project where the issue is reproduced so we could further investigate the reason for the error? You could open a support ticket for the issue and attach your project in it.
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 the blog feed