NDesk.Options: how to register required parameters correctly?

I am trying to utilize the OptionSet class in the following way:

  string resultsFileName = null;
  bool isHelp = false;

  var p = new OptionSet() {
    { "r=|resultsFile=",  "The file with the results", v => { resultsFileName = v; } }
    { "h|help", "Show this help", v => { isHelp = (v != null); } },
  };

  try
  {
    p.Parse(args);
  }
  catch (OptionException e)
  {
    Console.WriteLine("Invalid arguments: " + e.Message);
    ShowHelp(p);
    return;
  }

Thus, resultsFile option is expected to be required (according to documentation: http://www.ndesk.org/doc/ndesk-options/NDesk.Options/OptionValueType.html). However, when I run the program as is (i.e. with no command line arguments) the exception is not thrown. Please assist.

Update: It looks in debugger like when no command line options are provided, the private void AssertValid (int index) method will be never reached to throw new OptionException.