Laravel Cashier createAsStripeCustomer() needs argument

Following the docs here:

https://laravel.com/docs/5.7/billing#create-customers

(sadly anchor is broken so you will need to scroll to 'Creating Customers')

it says to register a user as a stripe customer you can run:

$user->createAsStripeCustomer();

However this dies with:

Too few arguments to function App\User::createAsStripeCustomer(), 0 passed

Seems it expects at array as an argument. Am I missing something or do the docs need updating?