I received a very odd error in Java.

import com.teamtreehouse.model.Song;
import com.teamtreehouse.model.SongBook;

public class Karaoke {

  public static void main(String[] args) {
    Song song = new Song(
      "Michael Jackson",
      "Beat It", 
      "https://www.youtube.com/watch?v=SaEC9i9Q0vk");
    SongBook songBook = new SongBook();
    System.out.printf("Adding %s %n", song);
    songBook.addSong(song);
    System.out.printf("There are %d songs. %n", songBook.getSongCount());
  }

}

package com.teamtreehouse.model;

public class Song {
  private String mArtist;
  private String mTitle;
  private String mVideoUrl;

  public Song(String artist, String title, String videoUrl) {
    mArtist = artist;
    mTitle = title;
    mVideoUrl = videoUrl;
  }

  public String getTitle() {
    return mTitle;
  }

  public String getArtist() {
    return mArtist;
  }

  public String getVideoUrl() {
    return mVideoUrl;
  }


  @Override
  public String toString() {
    return String.format("Song: %s by %s", mTitle, mArtist);
  }
}

package com.teamtreehouse.model;

import java.util.ArrayList;
import java.util.List;

public class SongBook{
  private List<Song> mSongs;

  public SongBook() {
    mSongs = newArrayList<Song>();
  }

  public void addSong(Song song) {
    mSong.add(song);
  }

  public int getSongCount() {
    return mSongs.size();
  }
}