Backup 1 Quiz with users

by Antoni Bertran Bellido.  

Hello I want to backup only one activity with users, but I can't set the setting users to 1 because is Locked 

base_setting::LOCKED_BY_PERMISSION

the code is like:

$cm         = get_coursemodule_from_id('', $cmid, 0, true, MUST_EXIST);
// backup the activity
$admin = get_admin();
$bc = new \backup_controller(\backup::TYPE_1ACTIVITY, $cm->id, \backup::FORMAT_MOODLE,
    \backup::INTERACTIVE_NO, \backup::MODE_IMPORT, $admin->id);
$backupsettings = array(
    'users' => 1,
    'role_assignments' => 0,
    'comments' => 0,
    'userscompletion' => 1,
    'logs' => 0,
    'grade_histories' => 0
);
foreach ($backupsettings as $name => $value) {
    $bc->get_plan()->get_setting($name)->set_value($value);
}
$backupid       = $bc->get_backupid();
$backupbasepath = $bc->get_plan()->get_basepath();
$bc->execute_plan();
$bc->destroy();

Thanks!