Paypal 500 Error: javax.validation.ValidationException - php

Question

On my first attempt to use PayPal's REST Api, I ran into a 500 error. I googled around a bit and wasn't able to find much helpful information on PayPal 500 errors, and I have no idea what would cause it.
The error is as follows.
500
javax.validation.ValidationException: HV000028: Unexpected exception during isValid call.[java.lang.NullPointerException]
exception 'PayPal\Exception\PayPalConnectionException' with message 'Got Http response code 500 when accessing https://api.sandbox.paypal.com/v1/payments/payment.' in /home/user/public_html/sandbox/vendor/paypal/rest-api-sdk-php/lib/PayPal/Core/PayPalHttpConnection.php:154
Stack trace:
#0 /home/user/public_html/sandbox/vendor/paypal/rest-api-sdk-php/lib/PayPal/Transport/PayPalRestCall.php(73): PayPal\Core\PayPalHttpConnection->execute('{"intent":"sale...')
#1 /home/user/public_html/sandbox/vendor/paypal/rest-api-sdk-php/lib/PayPal/Common/PayPalResourceModel.php(102): PayPal\Transport\PayPalRestCall->execute(Array, '/v1/payments/pa...', 'POST', '{"intent":"sale...', NULL)
#2 /home/user/public_html/sandbox/vendor/paypal/rest-api-sdk-php/lib/PayPal/Api/Payment.php(565): PayPal\Common\PayPalResourceModel::executeCall('/v1/payments/pa...', 'POST', '{"intent":"sale...', NULL, Object(PayPal\Rest\ApiContext), NULL)
#3 /home/user/public_html/sandbox/checkout.php(41): PayPal\Api\Payment->create(Object(PayPal\Rest\ApiContext))
#4 {main}
I also was unable to find a way to get a more verbose error message.
My code :
<?php
use PayPal\Api\Payer;
use PayPal\Api\Item;
use PayPal\Api\ItemList;
use PayPal\Api\Details;
use PayPal\Api\Amount;
use PayPal\Api\Transaction;
use PayPal\Api\RedirectUrls;
use PayPal\Api\Payment;
//include 'app.php';
require __DIR__ . '/vendor/autoload.php';
$apiContext = new \PayPal\Rest\ApiContext(
new \PayPal\Auth\OAuthTokenCredential(
'AfynNvA-2KwijS3pZ56jFPZwMxvObkwav8Vk4HMLc2foLahtCTPqzd_wfgt_wBtOCqqhLyk95T8LFUe0', // ClientID
'REDACTED' // ClientSecret
)
);
$price = 12;
$product = "Test";
$shipping = 0;
$total = $price + $shipping;
$payer = new Payer();
$payer->setPaymentMethod('paypal');
$item = new Item();
$item->setName($product)->setCurrency('USD')->setQuantity(1)->setPrice($price);
$itemList = new ItemList();
$itemList->setItems([$item]);
$details = new Details();
$details->setShipping($shipping)->setSubtotal($price);
$amount = new Amount();
$amount->setCurrency('USD')->setTotal($total)->setDetails($details);
$transaction = new Transaction();
$transaction->setAmount($amount)->setItemList($itemList)->setDescription("Test Payment")->setInvoiceNumber(uniqid());
$redirectUrls = new RedirectUrls();
$redirectUrls->setReturnUrl("REDACTED")->setCancelUrl("REDACTED");
$payment = new Payment();
$payment->setIntent('sale')->setPayer($payer)->setRedirectUrls($redirectUrls)->setTransactions([$transactions]);
try{
$payment->create($apiContext);
} catch (PayPal\Exception\PayPalConnectionException $ex) {
echo $ex->getCode(); // Prints the Error Code
echo "<br>";
echo $ex->getData(); // Prints the detailed error message
echo "<br>";
die($ex);
} catch (Exception $ex) {
die($ex);
}
echo ($payment->getApprovalLink());
?>
I'd be tempted to blame PayPal for this, as it is a 500, but it's my first use o the REST API so I assume I made a silly mistake. I've also attempted changing uniqid() to a static number, but that made no change. Should I submit a contact request to PayPal, and if so, how do I get the necessary information for that?