public function unsetUser($uid, array $properties, $eventTime=null)
    {
        $eventTime = $this->getEventTime($eventTime);
        if (empty($properties)) {
            throw new PredictionIOAPIError('Specify at least one property');
        }

        $json = json_encode([
            'event' => '$unset',
            'entityType' => 'user',
            'entityId' => $uid,
            'properties' => $properties,
            'eventTime' => $eventTime,
        ]);

        return $this->sendRequest('POST', $this->eventUrl, $json);
    }