Guzzle - HTTP Status Code 411

I am using Laravel 5 with Guzzle and I am attempting to send a message to an android device using Firebase cloud messaging. I have the JSON post message being built in a function with some of the variables passed to the function being used in the JSON message, however, I am getting a client error of HTTP status code 411 when sending. More specifically, Laravel is throwing:

ClientException in Middleware.php line 68: Client error: 411.  

I dont see anywhere where FCM requires that this header be set the error is occurring. My guess is that I have constructed the JSON message incorrectly, so here is the function that builds the POST message via GuzzleHttp:

    public function ProcessFirebaseNotification($recipientDevID){

    $client = new \GuzzleHttp\Client();
    $res = $client->request('POST', 'http://ift.tt/1TmHkUo', [
        'to' => $recipientDevID,
        'notification' => [
            'body' => 'Testing FCM',
            'title' => 'Testttt',
            ],
        'headers' => [
            'Content-Type' => 'application/json',
            'Authorization' => 'key=<-- My FCM Auth KEY -->'
        ]
    ]);

    return $res->getStatusCode();
}

If I lie and set

'Content-length' => '0'

Then the server throws a 400 Status code because it caught me up in the lie and I have made no progress. I saw other SO posts of people trying to send messages without a body and they get similar errors, but this POST is supposed to have a body.

Please let me know if any further information is needed. Thanks.

via Chebli Mohamed