I am not quite sure what is the exact error that you receive as you haven't provided the error's text.
I also do not know how exactly you have configured your RadToolTip - I created a simple test project, based on the provided information but unfortunately I was not able to reproduce the problem.
Please, modify the attached project in order to reproduce the problem, open a new support ticket and send it back to me along with the error's description. Once I receive it, I will do my best to help.
the Telerik team