Laravel 5.6 gives error when running mix on styles

Currently I'm trying to compile some css files into one using laravel mix.
When I run this:

mix.js('resources/assets/js/app.js', 'public/js')
    .js('resources/assets/js/bootstrap.js', 'public/js')
    .js('resources/assets/js/bootstrap-checkbox-radio-switch-tags.js', 'public/js')
    .js('resources/assets/js/bootstrap-notify.js', 'public/js')
    .js('resources/assets/js/bootstrap-selectpicker.js', 'public/js')
    .js('resources/assets/js/bootstrap.min.js', 'public/js')
    .js('resources/assets/js/charts/sparkline-charts.js', 'public/js/charts');

everything works fine. But when I attempt to run a css mix using either of these:

mix.styles([
      'resources/assets/css/bootstrap-datepicker3.css',
      'resources/assets/css/demo.css',
      'resources/assets/css/font-muli.css',
      'resources/assets/css/inspire.css',
      'resources/assets/css/themify-icons.css’], 'public/css/all.css');

or with out putting them in the square brackets like the above js it gives an error.

The error code I get is this:

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @ development: `cross-env NODE_ENV=development node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js`
npm ERR! Exit status 1

This just part of the code but the rest and also npm error code in debug.log isn't useful.

Any ideas on what I'm doing wrong?

via Chebli Mohamed