RegistrationException - "does not have a strong name"

I'm referencing a .NET DLL (which in turn references a lot of legacy COM DLLs) which is NOT strongly named. I'm referencing this from an NUnit project, which is also not signed.

However when running and trying to instantiate a class, I receive the error message:

RegistrationException: The class XXX does not have a strong name.

Why would this be?

Note that I'm not asking HOW to sign the assemb开发者_如何学运维ly (although that doesn't seem to work either) but rather WHY I'm being forced to sign it?

Any ideas are welcome!

Thanks Duncan