Typed property MailerSend\MailerSend::$bulkEmail must not be accessed before initialization

Question

I'm using Laravel 10 and Pest. A test that involves MailerSend client is giving me this error. I'm using the Mailsender PHP SDK:

Error: Typed property MailerSend\MailerSend::$bulkEmail must not be accessed before initialization in /opt/project/app/Actions/Email/SendEmail.php:33

This is the test:

it('sends an email', function () {
    Config::set('services.mailersend.key', 'foo');

    $this->mock(MailerSend::class, function (MockInterface $mock) {
        $mock->shouldReceive('bulkEmail->send')
            ->once()
            ->andReturn([
                'response' => 'success',
                'data' => [
                    'status_code' => 202,
                ],
                'body' => [
                    'bulk_email_id' => '123foo'
                ]
            ]);
    });

    actingAsAdmin();

    postJson(route($this->route, $this->payload))
        ->assertOk();
});

I've implemented the client in the boot of AppServiceProvider:

$this->app->bind(MailerSend::class, function () {
    return (new MailerSend([
        'api_key' => Config::get('services.mailersend.key')
    ]));
});

And I'm using it in an action, injecting it:

class SendEmail
{
    public function __construct(private readonly MailerSend $mailerSendClient)
    {
        //
    }

    /**
     * Send an email.
     */
    public function run(SendEmailDto $sendEmailDto): array
    {
        $bulkEmailParams = [];

        foreach($sendEmailDto->recipients as $recipient) {
            $bulkEmailParams[] = (new EmailParams())
                ->setFrom('[email protected]')
                ->setFromName('My Website')
                ->setRecipients($recipient)
                ->setSubject($sendEmailDto->subject)
                ->setHtml($sendEmailDto->text);
        }

        return $this->mailerSendClient
            ->bulkEmail
            ->send($bulkEmailParams);
    }
}

The endpoint works well, the emails are delivered, but the test won't pass.