Laravel testing

I'm currently testing a Laravel 5.5 package. The problem that is that when I try this:

this->post('/under/check', ['code' => 1235]);

The response is always page not found! When I try this in postman it's working and I receive a proper response?!?!

What could I be doing wrong?

It's an open source project so it's on github:

http://ift.tt/2xYsJM8