Cannot retrieve Stripe data on success page - php

Question

I have followed the Stripe documentation, trying to get the e-mail address of a customer but it fails with the following error:
PHP Fatal error: Uncaught exception 'Stripe\Exception\InvalidArgumentException'
with message 'The resource ID cannot be null or whitespace.' in /home/phuketto/public_html/stripe-php/lib/Service/AbstractService.php:99
Stack trace:
#0 /home/phuketto/public_html/stripe-php/lib/Service/Checkout/SessionService.php(90): Stripe\Service\AbstractService->buildPath('/v1/checkout/se...', NULL)
#1 /home/phuketto/public_html/ipnstripe.php(9): Stripe\Service\Checkout\SessionService->retrieve(NULL)
#2 {main}
thrown in /home/phuketto/public_html/stripe-php/lib/Service/AbstractService.php on line 99
In my create session script I am using:
'success_url' => 'https://www.example.com/ipnstripe.php?session_id={CHECKOUT_SESSION_ID}',
In my success_url I am trying to get the customer e-mail to be used further down the script but something goes wrong and the rest of the script is not executed:
require_once('./stripe-php/init.php');
$stripe = new \Stripe\StripeClient('sk_live_xxxxx');
$session = $stripe->checkout->sessions->retrieve($_GET['session_id']);
$customer = $stripe->customers->retrieve($session->customer);
$payeremail = $customer->email;