Unable to use spatie/laravel-newsletter

I'm looking to integrate Mailchimp into my application and allow my customers to subscribe for the mailing list. I came across the spatie/laravel-newsletter package which looked to be not only the perfection solution, but also extremely easy to implement (or so I thought).

1. I used composer to require the package and also published the config file.
2. I have a basic HTML form that passes information to my pages controller file.
3. When I try to test the feature, I get a vendor related error:

"Type error: Return value of Spatie\Newsletter\NewsletterList::getId() must be of the type string, null returned"

It's complaining about a specific part in the following file:

\vendor\spatie\laravel-newsletter\src\NewsletterList.php

public function getId(): string { return $this->properties['id']; }

I've tried updating composer but there's nothing to update or install. The package is meant to have very little installation steps so I really have no idea where to go from here.

Hopefully someone can point me in the right direction? :)