帮看下这段代码错在哪？

这是一段打开CAD的代码，编译能通过，但运行到m_AcadDoc=m_AcadApp.GetActiveDocument();时出错
    CLSID clsid;
    HRESULT h;
    CString str;
    h=::CLSIDFromProgID(L"AutoCAD.Application",&clsid);
    if(FAILED(h))
    {
        MessageBox("No Acad");
        return;
    }
    IUnknown *pUnknown=NULL;
    h=::GetActiveObject(clsid,NULL,&pUnknown);
    if(FAILED(h))
    {
        h=m_AcadApp.CreateDispatch(clsid);
        if(FAILED(h))
            MessageBox("Fail Acad");
        MessageBox("Ok Acad");
    }
    m_AcadDoc=m_AcadApp.GetActiveDocument();
    m_AcadMSpace=m_AcadDoc.GetModelSpace();
    m_AcadApp.SetVisible(TRUE);


变量在头文件中的定义是
    IAcadApplication m_AcadApp;
    IAcadModelSpace m_AcadMSpace;
    IAcadDocument m_AcadDoc;