Add reference gives "invalid 'assembly' metadata"

I have a .NET c++ dll I want to add and use in a c# script within a project. When I click on the script in the project, I can succesfully add the reference to the project and successfully compile in Mono. I found I need to use the file smcs.rsp to control the internal compiler in the Unity editor, and added this line:

-reference:"Assets\Plugins\MyDll.dll"

When trying to run, I get the compiler error "invalid 'assembly' metadata. Does anyone know where I should look to solve this. Should the internal compiler not behave just like the one that pops up when I click a script in the project?

This is for Windows desktop target.