Private npm registry

I am using v1.0.3 with a CLI utility that has been added to our private npm registry. The registry is defined both in my global (user) .npmrc and local CLI utility .npmrc but it is still coming back as undefined when I use:

const updateNotifier = require('update-notifier');
const pkg = require('./package.json');

// Checks for available update and returns an instance
const notifier = updateNotifier({pkg});

// Notify using the built-in convenience method
notifier.notify();

// `notifier.update` contains some useful info about the update
console.log(notifier.update);

Is there a step I am missing? Based on #32 I thought it should work with private registries. Maybe I'm wrong though.