An error occurred while validating. HRESULT = '80004005'

I've got an existing setup project that I'm trying to add some projects to.  I've added the projects to the solution, and they all build without error.  I'm having problems adding their output to the setup project.  When I do, I'm getting "An error occurred while validating.  HRESULT = '80004005'" error.  The dependencies for some of the newly added projects are not being detected and added to the setup project.  If I look at the properties dialog for the primary output, I can see under KeyOutput all the dependencies, but some of them are marked with an exclamation mark.  In the Dependencies property is listed "(None)."  I've tried adding the assemblies to the project folder, but still get the above error when I try to build.