PVS-Studio warning V547 for Miranda IM

typedef struct tagPROTOFILETRANSFERSTATUS
{
  ...
  unsigned __int64 currentFileSize;
  ...
} PROTOFILETRANSFERSTATUS;

//There is this place in the program code:
ft->std.currentFileSize = -1;

void __cdecl CJabberProto::FileReceiveThread(filetransfer* ft)
{
  ...
  if (ft->state==FT_DONE ||
      ( ft->state==FT_RECEIVING && ft->std.currentFileSize < 0 ))
  ...
}

This suspicious code was found in Miranda IM project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'ft->std.currentFileSize < 0' is always false. Unsigned type value is never < 0. jabber jabber_file.cpp 92

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/