“Empty” inherited COM interface is not marshalled properly

[igor.r...@gmail.com]

I've got an in-proc COM server (dll, built in MSVC10), which is used in a C# client. One of the interfaces is defined like this:
[
object,
uuid(some guid),
dual,
nonextensible,
helpstring("IDerived Interface"),
pointer_default(unique)
]
interface IDerived : IBase{
};

The co-class implements both IDerived and IBase (and some other interfaces):
[
uuid(some guid),
helpstring("SomeClass Class"),
noncreatable,
hidden
]
coclass SomeClass
{
[default] interface IDerived;
interface IBase;
// etc.
};

The co-class is apartment-threaded. When an MTA client thread acquires IDerived and tries to call any of its methods (inherited from IBase), it crashes somewhere in proxy/stub. In STA it works well, so I believe IDerived is not marshalled correctly. On the other hand, if I QI IBase explicitly or if I add a method to IDerived, everything works well.

Why isn't IDerived marshalled correctly?