Field ‚id‘ is required

I am trying to fire an application with the following code.

<?php
require_once('vendor/autoload.php');

$client = new \GuzzleHttp\Client();

$response = $client->request('POST', 'https://api.personio.de/v1/recruiting/applications', [
  'body' => '{"phase":{"type":"system"},"first_name":"Hans","last_name":"Wurst","email":"[email protected]","job_position_id":1170367}',
  'headers' => [
    'X-Company-ID' => 'XXXXXX',
    'accept' => 'application/json',
    'authorization' => 'Bearer XXXXXXXXXXXXXXX',
    'content-type' => 'application/json',
  ],
]);

echo $response->getBody();

But I always get the following error

{
  "error": {
    "reason": "Field 'id' is required"
  }
}

What is going wrong?