parameter problem in fire event

In my atl service with dual interface, i am calling the callback event in the
client and passing it a bstr value.
Fire_DownloadComplete(result); where result is a bstr.

this is how my Fire event is defined

HRESULT Fire_DownloadComplete(BSTR res)
{
    HRESULT hr = S_OK;
    T * pThis = static_cast<T *>(this);
    int cConnections = m_vec.GetSize();
    for (int iConnection = 0; iConnection < cConnections; iConnection++)
    {
    pThis->Lock();
    CComPtr<IUnknown> punkConnection = m_vec.GetAt(iConnection);
    pThis->Unlock();
    IDispatch * pConnection = static_cast<IDispatch *>(punkConnection.p);
    if (pConnection)
    {
    CComVariant avarParams[1];
    avarParams[0] = res;
    avarParams[0].vt = VT_BSTR;
    CComVariant varResult;
    DISPPARAMS params = { avarParams, NULL, 1, 0 };
                hr = pConnection->Invoke(1, IID_NULL, LOCALE_USER_DEFAULT,
DISPATCH_METHOD, &params, &varResult, NULL, NULL);
    }
    }
    return hr;
}

At the client side my sink method is defined as follows.

BEGIN_DISPATCH_MAP(CSink, CCmdTarget)
        DISP_FUNCTION(CSink, "DownloadComplete", DownloadComplete, VT_I4 , VTS_BSTR)
END_DISPATCH_MAP()

BEGIN_INTERFACE_MAP(CSink, CCmdTarget)//DIID__IXMathEvents
    //INTERFACE_PART(CSink, IID_ISink, Dispatch)
    INTERFACE_PART(CSink, DIID__IWebLinkHelperEvents, Dispatch)
END_INTERFACE_MAP()

/////////////////////////////////////////////////////////////////////////////
// CSink message handlers

HRESULT CSink::DownloadComplete(BSTR res)
{
    MessageBox(NULL,"Finished Downloading","Download Status",NULL);
    CString result(res);
    AfxMessageBox(result);
    isDownloadComplete = TRUE;
    return S_OK;
}

The callback event is getting called but the correct value of bstr is not
getting passed. I am getting a garbage value. what could be the problem?
Please advice.

Thanks in Advance!