Guzzle 401 response sending data by post

[dougth...@gmail.com]

I've been using Guzzle to get data from my API, but when I try and post data to the API I get the following error:

Fatal error: Uncaught exception 'GuzzleHttp\Exception\ClientException' with message 'Client error response [url] http://www.mywebsite.bar/rest/v4/foo/ [status code] 401 [reason phrase] Unauthorized : Invalid signature

The code I'm using to send the data looks like this

$request = $this->client->post('http://www.mywebsite.bar/rest/v4/foo/' ,
  array('Content-Type: application/x-www-form-urlencoded'),
  array());
$request->setBody($my_data); #set body!
$response = $request->send();

return $response;

Using the same client to get data works fine

$res = $this->client->get( $url);
return $res->getBody();

Using OAuth I've managed to post the data to the API like this

$my_data['name'] = 'Bob';
$my_data['age'] = 34;
$my_data['location[home]'] = 'foo';
$my_data['location[work]'] = 'bar';

$oauth->fetch( $url , $my_data , OAUTH_HTTP_METHOD_POST );

Sending the array to the API this way works fine.

I'm using guzzlehttp/oauth-subscriber for all the oauth side.

What am I doing wrong with Guzzle?

Thanks