L5.4 Styles Is Not A Function

I'm migrating a 5.3 app to 5.4. When running npm run dev, I get

/home/michael/Code/oispark/webpack.mix.js:36
    .styles([
     ^

TypeError: mix.styles is not a function

This is my css section:

mix.styles([
        'resources/assets/css/font-awesome.css',
        'resources/assets/css/styles.css',
        'resources/assets/css/custom.css',
        'resources/assets/css/responsive.css',
    ], 'public/css/styles.css');

There's nothing in the docs that say you have to do anything else: https://laravel.com/docs/5.4/mix#plain-css

Does anyone have any ideas?