Using Carbon dates in Laravel/Lumen

I am building a webservice based on the latest version of Lumen to use with a Guzzle 6 client. The webservice is working but when I acces a carbon date on the client with:

$newsitem->publish_start->formatLocalized('%A %d %B %Y')

I get the following error:

Call to a member function formatLocalized() on a non-object

In my webservice 'News' model part I have:

protected $dates = [
    'publish_start',
    'publish_end'
];

If I remove these and access the date with

date('Y/m', strtotime($newsitem->publish_start))

it all works fine.

What am I missing here? How do I get Carbon dates through the webservice? Do I have to activate Carbon manually on Lumen?

via Chebli Mohamed