MOZILLA SOAP API via XPCOM in C++

Hello,

I'm quite new to XPCOM and I need an example using Mozillas SOAP
functions in a C++ program. The Program itself is not intended to be a
XPCOM library, but it should get the MOZILLA SOAP functions via XPCOM. I
managed to create an instance of nsSOAPCall, creating the necessary
params, calling Encode(...). 
When I'm calling Invoke() like this:  
rv = myc->Invoke( (nsISOAPResponse**) &myr ); 
where myc is a nsCOMPtr to the nsSOAPCall instance, and myr is a
nsCOMPtr to a nsSOAPResponse instance, it only delivers a zero pointer,
i.e. myr is nsnull.
A similar example in javascript works perfectly in the Browser. 
Can anybody tell me, what to do exactly to use nsSOAPCall in C++? Maybe
there exists a sample? If necessary, I can post my code.

Thanks Ernst