"Context does not exist" error

hi folks,

I'm under Moodle 2.3

I'm trying to enrol a student to a course using a rest web service.

I'm receiving the message "Context does not exist" as an invalidparameter.

Here is my code. 

******************************************

/// SETUP - NEED TO BE CHANGED
$token = 'd6b89e0ebdd2667e16830e0c8b1093fa';
$domainname = 'http://ludirm.medzone.fr';
$functionname = 'enrol_manual_enrol_users';

//////// ////////

$enrolment = array(
'roleid' => 5,
'userid' => 24,
'courseid' => 69
);
$enrolments = array( $enrolment );
$params = array( 'enrolments' => $enrolments );

/// REST CALL
header('Content-Type: text/plain');
$serverurl = $domainname . '/webservice/rest/server.php'. '?wstoken=' . $token . '&wsfunction='.$functionname;
require_once('./curl.php');
$curl = new curl;

$resp = $curl->post($serverurl, $params);
print_r($resp);
print_r($params);

*****************************************

If anybody has any idea, I'll be greatful. I've been trying to fix it for couple of hours now.

Many thanks in advance for your support.

Lilian