laravel 5.1 queues undefined function in handle() but working in constructor

I use Laravel 5.1 queues to process large images. I've a very strange behavior since when I call a function "imagettfbbox" in the constructor it works. But obviously I need to make it work in the "handle" but there I got an error.

public function __construct()
{
    //TEST
    $font_path = public_path('/fonts/roboto/Roboto-Thin.ttf');

    imagecreate(10,10); //works!
    imagettfbbox(10, 0, $font_path, 'test'); //works!
}

public function handle() //GenerateImage $generator, Image $img
{
    //TEST
    print 'OK'; //gets printed
    $font_path = public_path('/fonts/roboto/Roboto-Thin.ttf');

    imagecreate(10,10); //works!
    imagettfbbox(10, 0, $font_path, 'test'); //CRASHES!
}

I get the 'OK' printed and then the error "Call to undefined function App\Jobs\imagettfbbox()". It is a very strange behavior since some image functions work other not. I've GD installed and everywhere outside handle the code works. Any clue what I'm missing here?

via Chebli Mohamed