Issues with using nightwatch slack reporter

I'm trying to set up my nightwatchjs tests to report to Slack, using the nightwatch-slack-reporter nodejs package.

When I run my nightwatchjs test from the command line, using the following;

npm run test:default /current_functionality/PAR-5139.js --reporter node_modules/nightwatch-slack-reporter/lib/reporter.js

I get the following error;

✖ TypeError: Cannot read property 'slack_webhook_url' of undefined

My globals file looks like this;

var options = {
  slack_message: function(results, options) {
    return {
      text: 'Test completed, passed ' + results.passed + ', failed ' + results.failed,
      username: 'Nightwatch',
      icon_emoji: ':ghost:'
    };
  },
  slack_webhook_url: 'https://hooks.slack.com/services/................'
};

module.exports = {
  reporter: (require('nightwatch-slack-reporter')(options))
};

So firstly, is this anything obviously wrong with my globals setup?

Also, I'm wondering if my webhook url is correct?

I copied and pasted it from my slack app, but is this the right place to get the webhook from, or should I be getting it from the slack instance that the test is reporting to?

Despite this failure, it does actually report the failure to Slack, as below;

but the test is not running/passing.

Any help would be greatly appreciated.

Thanks