PVS-Studio warning V547 for ACE

ACEXML_Char*
ACEXML_Parser::parse_reference_name (void)
{
  ACEXML_Char ch = this->get ();
  if (!this->isLetter (ch) && (ch != '_' || ch != ':'))
    return 0;
  ...
}

This suspicious code was found in ACE project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'ch != '_' || ch != ':'' is always true. Probably the '&&' operator should be used here. ACEXML_Parser parser.cpp 1905

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