BeforeNavigate2 and PostData

Hi, I'm capturing the BeforeNavigate2 event of a web browser control. I want to change the PostData and resend the request (Navigate2). It gets resent, but the post data becomes empty. Here's the event:

Code:

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void BeforeNavigate2(IDispatch *pDisp, VARIANT * url,VARIANT * Flags,VARIANT * TargetFrameName, VARIANT * PostData,VARIANT * Headers,  VARIANT_BOOL * Cancel)
{
        static BOOL isReNavigate = 1;
        if(isReNavigate == 1)
        {
                IWebBrowser2 *iweb = NULL;
                *Cancel = VARIANT_TRUE;
                isReNavigate = 0;
                pDisp->lpVtbl->QueryInterface
                        (pDisp, &IID_IWebBrowser2, (void**)&iweb);
                iweb->lpVtbl->Navigate2
                        (iweb, url, Flags, TargetFrameName, PostData, Headers); //the browser opens the url, but no postdata is sent, I've checked that there is suitable data in PostData and it works if I'm not resending (but I need to edit the PostData).
        }
        else
        {
                isReNavigate = 1;
        }
}

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If that's the wrong way, how would you advise to do it? Thanks