Indirect modification of overloaded property when building a PHPUnit test for Braintree webhook

I am trying to build a test with PHPUnit to test my Braintree hook. I have this test case which uses the dataProvider below:

/**
 * @covers SubscriptionExpiredHook::processHook()
 * @dataProvider dataProvider
 */
public function testProcessHook($subscriptionId)
{
    $subscriptionExpiredHook = new SubscriptionExpiredHook($this->siteManager);

    $notification = $this->getMockBuilder('Braintree\WebhookNotification')
        ->disableOriginalConstructor()
        ->getMock();

    $notification->subscription->id = $subscriptionId;

    $router = $this->getRouterMock();

    $subscriptionExpiredHook->setSubscription(new Subscription($router));

    $response = $subscriptionExpiredHook->processHook($notification);

    $this->assertEquals(200, $response->getStatusCode());
}

public function dataProvider()
{
    return [
        [
            'subscription_id' => 'CRM1872',
        ]
    ];
}

The line that is causing the problem is this:

$subscriptionId = $hook->subscription->id;

...and this is the error I get when I run the test:

1) CRMPiccoBundle\Tests\Braintree\Hooks\SubscriptionExpiredHookTest::testProcessHook with data set #0 ('CRM1872') Indirect modification of overloaded property Mock_WebhookNotification_e65192ec::$subscription has no effect

Is there a way that I can set the subscription id to allow this test to proceed or am I looking at it the wrong way?