C++ program receiving HOST_E_CLRNOTAVAILABLE when calling a C# interface

I have a C++ program calling an interface I've implemented in C#. I'm consistently getting the error 0x80131023 HOST_E_CLRNOTAVAILABLE which is defined as CLR has been disabled due to unrecoverable error. The c++ program doesn't produce this error unless my program is being called but I can't see any of my functions in the stack trace. Does anyone have any tips on how go about identifying the problem?

EDIT: I'm using late binding to create an IDispatch interface.

 IUnknown* pUnknown;
 hRC = CoCreateInstance(
        MyProgID,                  
        0,                          
        CLSCTX_ALL,                 
        IID_IUnknown,               
        (void**) &pUnknown);        
hRC = pUnknown->QueryInterface(
        IID_IDispatch,              
        (void**) &_pService );      
pUnknown->Release();  

Then invoking with:

hRC = _pService->Invoke(
        _DispIDs[nDispIDIndex], IID_NULL, LOCALE_USER_DEFAULT,
        METHOD_TO_CALL, &Disp, &VarResult, NULL, NULL ); 

It calls most of the methods successfully with no error. When the Close method is called it returns the error mentioned.

The most likely failure mode here is that one of your calls tripped a managed exception that wasn't caught and caused the CLR to shut down. This might have happened in a thread that was started by the managed code so it won't get reported back to you with an HRESULT. That leaves a trace, the Output window ought to contain a notification for it, a "first chance exception" for exception type 0xe0534f4d.

Use a managed debugger to troubleshoot. VS has one: Project + Properties, Debugging, Debugger type setting. You can force a automatic break with Debug + Exceptions, tick the Thrown box for CLR exceptions.