Firebase FCM - 411 Error. POST requests require a Content-length header. That’s all we know

I am trying to send notifications to devices using FCM. On the server-side, i have got the authorisation token through the FCM Admin SDK in python ($auth_token), and the device $token. On running the code, I get the following function:

function sendNotification($token, $title,$body){

    $command = escapeshellcmd('/usr/bin/python3 api/fcm.py');
    $auth_token = shell_exec($command); 

    // set request data
    $request_data = array(
    'message' => array(
        'token' => $token,
        'notification' => array(
            'body' => $body,
            'title' => $title
            )
        )
    );

    $fields = json_encode($request_data);
    $request_length = strlen($fields);
    print($request_length);

    // set headers
    $headers = array(
        'Content-Type: application/json',
        'Authorization: Bearer ' . $auth_token,
        'Content-Length: '. $request_length
    );

    $url = 'https://fcm.googleapis.com/v1/projects/black-queen/8eff3/messages:send';

    $ch = curl_init ();
    curl_setopt ( $ch, CURLOPT_URL, $url );
    curl_setopt ( $ch, CURLOPT_POST, true );
    curl_setopt ( $ch, CURLOPT_HTTPHEADER, $headers );
    curl_setopt ( $ch, CURLOPT_RETURNTRANSFER, true );
    curl_setopt ( $ch, CURLOPT_POSTFIELDS, $fields );
    curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);

    $result = curl_exec ( $ch );
    echo $result;

    // check for errors
    if(curl_errno($ch)) {
        print('Error: ' . curl_error($ch));
    }

    // close cURL session
    curl_close($ch);

    // print response
    print($response);
}

However, I am getting the following error: "411. That’s an error. POST requests require a Content-length header. That’s all we know."

In case anyone else faces this issue, the issue on my end was white space. I put trim() around all the header inputs (in particular the $auth_token for me) which fixed it