PVS-Studio warning V530 for QT

int main(int argc, char **argv)
{
  ....
  QByteArray arg(argv[a]);
  ....
  arg = arg.mid(1);
  arg.toLower();
  if (arg == "o")
  ....
}

This suspicious code was found in QT project by PVS-Studio static code analyzer.
Warning message is:
V530 The return value of function 'toLower' is required to be utilized. main.cpp 72

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/