PVS-Studio warning V547 for UltimateTCPIP

int CUT_MimeEncode::Encode7bit(....)
{
  ...
  char buf[1000];
  ...
  for ( t=0; t<bytesRead; t++ ) {
    if (buf[t]==0 || buf[t]>127) {
      retval = UTE_ENCODING_INVALID_CHAR;
      break;
    }
  ...
}

This suspicious code was found in UltimateTCPIP project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'buf[t] > 127' is always false. The value range of signed char type: [-128, 127]. UTImap4 utmime.cpp 320

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