PVS-Studio warning V514 for OpenMS

CompressedInputSource::CompressedInputSource(
  const String & file_path, const char * header,
  MemoryManager * const manager)
  : xercesc::InputSource(manager)
{
  if (sizeof(header) / sizeof(char) > 1)
  {
    head_[0] = header[0];
    head_[1] = header[1];
  }
  else
  {
    head_[0] = '\0';
    head_[1] = '\0';
  }
  ....
}

This suspicious code was found in OpenMS project by PVS-Studio static code analyzer.
Warning message is:
V514 Dividing sizeof a pointer 'sizeof (header)' by another value. There is a probability of logical error presence. compressedinputsource.c 52

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/