NTLM: QNetworkAccessManager::put() does not work when authentication is required

I've tried uploading a file to an IIS6 WebDAV server using QNetworkAccessManager::put().
On an open server this performs flawless however when an authentication is reqired QNetworkReply::NetworkError returns QNetworkReply::UnknownContentError. The return from the server is 400 (Bad Request). Uploading to the server using windows works.

I believe the content-length in frame 15 (NTLMSSP_NEGOTIATE) is incorrect?
Qt seems to insert the length of the original data while the IIS expects 0.