Laravel Cashier Webhook Error "Class name must be a valid object or a string"

Working in Laravel 5.8, setting up subscriptions using Cashier and Stripe. Have setup my webhook endpoint and configured it on the Stripe side. When I send a test webhook, I get this error in my log:

[2021-11-03 21:59:33] local.ERROR: Class name must be a valid object or a string {"exception":"[object] (Symfony\Component\Debug\Exception\FatalThrowableError(code: 0): Class name must be a valid object or a string at /opt/bitnami/frameworks/laravel/vendor/laravel/cashier/src/Cashier.php:69)

Have tried clearing the config cache with artisan. Also tried declaring the cashier model in my .env file. Neither worked.

UPDATE:

Have tried a bunch of stuff to fix it, but this is the only thing that seems to work. I changed this:

$model = config('cashier.model');

To this:

$model = \App\User::Class;

In Cashier.php and the webhook works. Obviously editing things in the vendor folder isn't good, but I'm not sure what else to do.