PVS-Studio warning V610 for FFMpeg

static int mc_subpel(....)
{
  ....
  mx = motion_x & ~(-1 << s->mv_precision);
  ....
}

This suspicious code was found in FFMpeg project by PVS-Studio static code analyzer.
Warning message is:
V610 Undefined behavior. Check the shift operator '<<. The left operand '-1' is negative. diracdec.c 1358

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/