Ok I've found a solution to this particular error message on here already. But my case is slightly different. There are no "non-public" or "static" methods in my code. All are public. What I'm trying to do is pass a FrameworkElement (more specifically a web browser control) that was created in one process over to another process for display and use. Also I'm not using (and would to avoid using) any of the framework 3.5 addin stuff. Fails at the following line everytime. fe = FrameworkElementAdapters.ContractToViewAdapter(tab.ReturnBrowserObject) tab.ReturnBrowserObject returns an INativeHandleContract which the above line is suppose to convert to a FrameworkElement.