QString::toStdString() returns NULL on Windows x64

This function returns NULL on Windows. It works as expected on Mac and GNU/Linux.

 std::string
get_for_filename ()
{
  QDateTime datetime = QDateTime::currentDateTime ();
  QString   str_datetime =
    datetime.toString (QString::fromUtf8 ("yyyy-MM-dd_HH-mm-ss"));

  return str_datetime.toStdString ();
}

I stepped through the debugger in visual studio and I can see the correct string value inside my `str_datetime` variable. The result of this function is NULL according to the debugger, and I also get a memory-related fatal error when I attempt to use that result.

If I change the last line to `return str_datetime.toUtf8 ().data ();` then this function returns the expected string and everything works.