At first glance it appears that you are not doing anything wrong. I am shooting in the dark but one possible cause might be the line:
Try modifying it so it becomes something like:
If this does not help, could you please give us more info so we can better determine what is causing the problem. Meanwhile, you can take a look at the attached sample project for further reference. Have a look at it and if you have additional questions let us know.
the Telerik team