mix.copyDirectory is not a function in Laravel Elixir 5

According to the docs, in order to maintain the directory's structure I should use copyDirectory but I obtain the following:

/webpack.mix.js:16
   .copyDirectory('resources/assets/bower', 'public/js');
    ^

TypeError: mix.js(...).sass(...).copyDirectory is not a function

This is my webpack file:

const { mix } = require('laravel-mix');

mix.js('resources/assets/js/app.js', 'public/js')
   .sass('resources/assets/sass/app.scss', 'public/css')
   .copyDirectory('resources/assets/bower', 'public/js');

Any ideas? Thanks

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