PVS-Studio warning V610 for FlightGear

inline void StandardManipulator::setRelativeFlag(
  int index, bool value )
{
  if( value )  _relativeFlags |= (0x01 << index);
  else  _relativeFlags &= (~0x01 << index);
}

This suspicious code was found in FlightGear project by PVS-Studio static code analyzer.
Warning message is:
V610 Undefined behavior. Check the shift operator '<<. The left operand '~0x01' is negative. standardmanipulator 185

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/