Addin will not load when I pass Interop Outlook objects to objects to ctors outside the addin

I am building an Outlook addin using C#. I have created an Interop version
of the Outlook 9 dll, as referred to in the "literature", and have taken
precautions to fix the bug with event handling which can cause Interop
Outlook applications to crash.

Everything had been working fine using this library until I started passing
Interop Outlook objects to other objects in my code. Methods belonging to my
addin Connect object were fine, but methods belonging to other objects
caused the addin not to load at all.

For instance, if I have the Connect.Foo(Interop_Outlook.MAPIFolder folder)
method, that's fine, but if I define a Foo.ctor(Interop_Outlook.MAPIFolder
folder) method (constructor for a Foo object), the program compiles, but the
addin doesn't load on the target machine at all. It is as if an exception is
thrown while loading the addin which I never see. (n.b. I don't even have to
call the Foo.ctor - it is enough just to define it for the addin not to
load)

Any ideas on this would be greatly appreciated.
Regards,
Ben