No valid controls were found in the assembly

That is the error message i get when trying to add a component to the Web Matrix toolbox.

I know the dll file does work because if i manually add the appropriate code i get the control do display and work in a browser fine.

The control is open source, so hopefully i'd like to be able to add appropriate code etc. to get MS Web Matrix to recognise the component within the dll so that i can use the visual designer to preview my page, as i'm adding the control(s).

FYI it's written in vb.net.

Any help would be much appreciated.