Calling back-and-forth across process

Hi guys,

I have the following scenario:
A COM object in a dll calls into a COM object in an exe.
The two objects ultimately implement the same base interface (which,
btw implements IDispatchEx).

So let's say I have
interface IGenericNode : IDispatchEx
{
   [id(1)] HRESULT GetPath(BSTR *path);
   [id(2)] HRESULT SetupPath(IGenericNode *parent, BSTR name);
}

interface IElement : IGenericNode
{
   [id(3)] ...
}

The dll COM object "Host" implements IElement, while
the exe COM object "Globals" (created by the dll at runtime) implements
IGenericNode.

Now here's the funky part.

The dll object is doing this:
....
BSTR name = SysAllocString(L"Globals");
spGlobalsExe->SetupPath(this,name);
(where spGlobalsExe is of type CComQIPtr<IDOMNodeGeneric>)
....

And on the exe side, SetupPath is making a call back into the dll.
HRESULT SetupPath(IGenericNode *parent, BSTR name);
BSTR parentPath;
HRESULT hr = parent->GetPath(&parentPath);

The call into the exe succeeds, and the "name" variable is in fact
transmitted.

However, the call back into the dll - to GetPath - fails: parentPath is
left uninitialized, and the hr is '0xc0000005: The instruction at "0x'
which I assume means that an exception is occurring. Furthermore, the
call does not even make it back into the dll (a breakpoint GetPath() in
the dll does not get hit).

I'm assuming this has to do with marshalling, so I'll add the following
information:
1) I do have two proxy/stub dlls - for each the dll and the exe
2) both CoClasses I mention above implement
CComObjectRootEx<CComSingleThreadmodel>

Can anyone help? Thanks very much in advance.

Eric