PVS-Studio warning V610 for UCSniff

#define FP_HOST_NONLOCAL   1<<1

FUNC_DECODER(decode_ip)
{
  ...
  PACKET->PASSIVE.flags &= ~FP_HOST_NONLOCAL;
  ...
}

This suspicious code was found in UCSniff project by PVS-Studio static code analyzer.
Warning message is:
V610 Undefined behavior. Check the shift operator '<<. The left operand '~1' is negative.  ec_ip.c 187

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/