changing a collection into an array

I am running the code below. I thought that json_output would hold an array of the data, but instead it gets NULL. Why? 
 $stripe = new \Stripe\StripeClient($_SESSION['STRIPE_SECRET_KEY']);
        $js = $stripe->customers->all(['email' => $_SESSION['userid']]);
        $json_output=json_decode($js,true);

        $cust_id = $json_output->lastResponse->json->data[0]->id;
        $_SESSION['stripe_customer_number'] = $cust_id;
$cust_id also gets NULL.

proof that an old dog can learn new tricks