PVS-Studio warning V610 for GDCM

bool ImageCodec::DoOverlayCleanup(....)
{
  ....
  // nmask : to propagate sign bit on negative values
  int16_t nmask = (int16_t)0x8000;
  nmask = nmask >>
          ( PF.GetBitsAllocated() - PF.GetBitsStored() - 1 );
  ....
}

This suspicious code was found in GDCM project by PVS-Studio static code analyzer.
Warning message is:
V610 Unspecified behavior. Check the shift operator '>>. The left operand 'nmask' is negative. gdcmimagecodec.cxx 397

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