how to implement nsIWepProgressListener in C++

Hello,

I have the following class declaration:

class MyLocationListener: public nsIWebProgressListener
{
public:
 MyLocationListener();
 virtual ~MyLocationListener();
 NS_DECL_ISUPPORTS
 NS_DECL_NSIWEBPROGRESSLISTENER
};

and the corresponding implementation. I am using this class in the following way:

MyLocationListener * listener = new MyLocationListener();

nsCOMPtr<nsIWeakReference> weakref = do_GetWeakReference(listener, &retval);
CHECKERR();
retval = browser->AddWebBrowserListener(weakref, NS_GET_IID(nsIWebProgressListener));
CHECKERR();

however, the do_GetWeakReference call fails with an NS_ERROR_NO_INTERFACE error code. Can someone point out to me how to fix that? I tried to add a nsIWeakReference superclass to the class decl, including the NS_DECL_xx macros, but that yielded compile errors that I was notable to decipher. Maybe someone has a quick hint, or link..

TIA,
Christian