Error Rendering Control

[SAL]

Does anyone know what causes the following issue at design time in VS2005
with regards to Custom Controls:

Error Rendering Control - UpdatePanel1
An unhandled exception has occurred.
Specified method is not supported.

Again, this is at design time, and the ASP project still compiles without
errors. The page and control displays at run time as it should. This is
annoying issue that I would like to know what causes it. I've registered my
control in the source as:

<%@ Register Src="Controls/MyControl.ascx" TagName="MyControl"
TagPrefix="TEST" %>

Thanks for you help in advance.