Buttons work until I use the "next" button to play the next song

/////////////////////////////////Songs
function Song(title, artist, duration) {
    this.title = title;
    this.artist = artist;
    this.duration = duration;
    this.isPlaying = false;
}

Song.prototype.play = function() {
    this.isPlaying = true;
};

Song.prototype.stop = function() {
    this.isPlaying = false;
};

Song.prototype.toHTML = function() {
    let htmlString = '<li'
    if (this.isPlaying) {
        htmlString += 'class="current"'
    };
    htmlString += '>'
    htmlString += this.title;
    htmlString += '-';
    htmlString += this.artist;
    htmlString += '<span class="duration">'
    htmlString += this.duration;
    htmlString += '</span></li>';

    return htmlString;
};

///////////////////////////////////PlayList
function Playlist() {
    this.songs = [];
    this.nowPlayingIndex = 0;
}

Playlist.prototype.add = function(song) {
    this.songs.push(song);
};

Playlist.prototype.play = function() {
    let currentSong = this.songs[this.nowPlayingIndex];
    currentSong.play();
};

Playlist.prototype.stop = function() {
    let currentSong = this.songs[this.nowPlayingIndex];
    currentSong.stop();
};

Playlist.prototype.next = function() {
    this.stop();
    this.nowPlayingIndex++;
    if (this.nowPlayingIndex === this.songs.length) {
        this.nowPlayingIndex === 0;
    }
    this.play();
};

Playlist.prototype.renderInElement = function(list) {
    list.innerHTML = " ";
    for (let i = 0; i < this.songs.length; i++) {
        list.innerHTML += this.songs[i].toHTML();

    }
};

//////////////////////////////App
let playlist = new Playlist();

let hereComesTheSun = new Song("Here Comes the Sun", "The Beatles", "2:54");
let walkingOnSunshine = new Song("Walking on Sunshine", "Katrina and the Waves", "3:43");

playlist.add(hereComesTheSun);
playlist.add(walkingOnSunshine);

let playlistElement = document.getElementById("playlist");
playlist.renderInElement(playlistElement);

var playButton = document.getElementById('play');
playButton.onclick = function() {
    playlist.play();
    playlist.renderInElement(playlistElement);
}

var nextButton = document.getElementById('next');
nextButton.onclick = function() {
    playlist.next();
    playlist.renderInElement(playlistElement);
}

var stopButton = document.getElementById('stop');
stopButton.onclick = function() {
    playlist.stop();
    playlist.renderInElement(playlistElement);
}