Getting a console error about why my songs aren't displaying. Not able to find the mistake

// My app.js code
var playlist = Playlist();

var moneyAintAThing = new Playlist("Money Aint A Thing", "Jay-Z", "3:45");
var hardKnockLife = new Playlist("Hard Knock Life", "Jay-Z", "3:30");

playlist.add(moneyAintAThing);
playlist.add(hardKnockLife);

var 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);
}

// My song.js code
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() {
  var 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;
};

//My playlist.js code
function Playlist() {
  this.songs = [];
  this.nowPlayingIndex = 0;
}

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

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

Playlist.prototype.stop = function(){
  var 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(var i = 0; i < this.songs.length; i++) {
    list.innerHTML += this.songs[i].toHTML();
  }
};