Mayan EDMS API - Cannot add a document to a cabinet - maya-api

Question

Using Mayan EDMS, I am unable to add a document to a cabinet.
I am using PHP to send, create and upload a document, which works well.
Thereafter, trying to add the document to the cabinet results in the following error
{"detail":"Not found."}
I am at a loss on how to proceed as the API documentation is not clear about the request body and fails on the execute in the swagger documentation.
// Add document to cabinet
$cabinet = $params['cabinet'];
$data = array (
'document' => [$document_id], // values must be a list []
);
$post_data = json_encode($data);
$request_url = 'http://example.com/api/v4/cabinets/' . $cabinet . '/documents/add/';
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $request_url);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $post_data);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLINFO_HEADER_OUT, true);
$headers = [];
$headers[] = 'Accept: application/json';
$headers[] = 'Content-Type: application/json';
$headers[] = 'Content-Length: ' . strlen($post_data);
$headers[] = 'Authorization: Basic YWRtaW46QndLcFNaQnJURDI3MzN5';
$result = curl_exec($ch);
// $result returns : {"detail":"Not found."}
curl_close($ch);