Call to undefined function Milon\\Barcode\\app()

[nightwalkeryao]

Hello,
I am using this package outside of a Laravel project to generate an image for a PDF417 code but it fails with error Call to undefined function Milon\\Barcode\\app(),
file: [...]\\vendor\\milon\\barcode\\src\\Milon\\Barcode\\DNS2D.php, line: 183

My code looks like

use Milon\Barcode\DNS2D;

// ...
$code = "My text goes here";
$pdf417 = new DNS2D();
$raw_image = base64_decode($pdf417->getBarcodePNG($code, "PDF417,8", 3, 3));
// ...

I do not have any app function in my codebase.