this.songs[i].toHTML not a function

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();
  }
};

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;
};

var playlist = new Playlist();

var goodnightIrene = ("Goodnight Irene", "Hank Williams", "2:32");
var electricFeel = ("Electric Feel", "MGMT", "4:02");

playlist.add(goodnightIrene);
playlist.add(electricFeel);

var playlistElement = document.getElementById("playlist");

playlist.renderInElement(playlistElement);