public function addGenre($genre)
    {
        if (is_null($genre)) {
            throw new \InvalidArgumentException('Genre cannot be null');
        }

        if (!is_string($genre)) {
            throw new \InvalidArgumentException('Genre must be string');
        }

        if (!in_array($genre, $this->genres)) {
            $this->genres[] = $genre;
        }

        return $this;
    }