public function setLogger($logger = null)
    {
        if (! (is_null($logger)
            || $logger instanceof LoggerInterface)) {
            throw new InvalidArgumentException(
                'Argument to ' . __METHOD__ .
                " must be a valid Logger Interface (aka. Monolog), " .
                get_class($logger) . ' given.'
            );
        }

        $this->logger = $logger;
    }