Option function and default parameter is not executed

In source diving the arguments passed to command.option, it appears that it accepts a 3rd and 4th parameter for defining a function and default value for the option, but it appears that these aren't actually being executed. In this test case, I was unable to get a default value or have the function execute. Am I doing something wrong, or is this just not supported yet?

#! /usr/bin/env node

let Vorpal = require('vorpal');
let cli = new Vorpal();

cli
  .command('test')
  .description('A test')
  .option('--url <url>', 'Some url', function(val) { cli.log('VALUE!', value); }, 'http://some.url')
  .action(function(args, done) {
    this.log('URL:', args.options.url);
    done();
  });

cli.parse(process.argv);

$ ./cli test
URL: undefined

$ ./cli test --url other.url
URL: other.url