Error loading type library DLL. (Exception from HRESULT: 0x80029C4A(Type_E_CANTLOADLIBRARY))

I'm developing a c# desktop app on a customer's dev machine. Part of the app involves using a third party dll to communicate with another application installed on the box.

My app builds fine and even executes to a certain degree but whilst debugging I noticed I was getting the error

"Error loading type library DLL. (Exception from HRESULT: 0x80029C4A(Type_E_CANTLOADLIBRARY))" 

when I was instantiating the third party object and I think it may be setting up errors further down the line.

I looked into the exception Source and 'mscorlib' is referenced - not sure what relevance that would have?

Does anyone have any pointers?