Heroku cannot find private npmjs modules

Question

I'm new to Heroku and I've got an application that has private npmjs dependencies listed in its package.json file like this:

...
dependencies: {
    "@myscope/somemodule": "0.0.1", 
    ....
}

And I've got my .npmrc file setup like so:

//registry.npmjs.org/:_authToken=${NPM_TOKEN}

Where NPM_TOKEN has been set as a local environment variable and as a configuration variable in Heroku and on Codeship

The Heroku app fails to deploy both locally and on Codeship.

Specifically, I get a 404 error code that says somemodule cannot be found in the registry.

Further note, I can run a local instance of the Heroku app

I'm following this article:

http://blog.npmjs.org/post/118393368555/deploying-with-npm-private-modules

Am I missing a step?

Thanks!