PVS-Studio warning V567 for Word11a

CmdBitmap()
{
  static int  iBitmap = 0;
  ....
  iBitmap = ++iBitmap % MAXBITMAP;
}

This suspicious code was found in Word11a project by PVS-Studio static code analyzer.
Warning message is:
V567 Undefined behavior. The 'iBitmap' variable is modified while being used twice between sequence points. ddedit.c 107

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/