Relay Mutation - Error: "message": "Cannot read property 'id' of undefined"

PlaylistSchema.js

exports.getPlaylistById = (root, {id}) => {
  return new Promise((resolve, reject) => {
    Playlist.findOne({id:id}).populate('songs').exec((err, res) => {
      err ? reject(err) : resolve(res);
    })
  });
};

exports.addSong = ({id, youtubeLink}) => {
  return new Promise((resolve, reject) => {
    Playlist.findOne({id: id}).populate('songs').exec((err, res) => {
      err ? reject(err) : resolve(res);
    })
  });
};

schema.js

var {nodeInterface, nodeField} = nodeDefinitions(
  (globalId) => {
    var {type, id} = fromGlobalId(globalId);
    if (type === 'User') {
      return User.getUserById({id: id});
    } else if (type === 'Hobby') {
      return Hobby.getHobbyById({id: id});
    } else if (type === 'Playlist') {
      return Playlist.getPlaylistById({id: id});
    } else {
      return null;
    }
  },
  (obj) => {
    if (obj.type == "user") {
      return UserType;
    } else if (obj.type == "hobby")  {
      return HobbyType;
    } else if (obj.type == "playlist")  {
      return PlaylistType;
    }else {
      return null;
    }
  }
);

var {
  connectionType: PlaylistConnection,
  edgeType: PlaylistTypeEdge,
} = connectionDefinitions({
  name: 'Playlist',
  nodeType: PlaylistType,
});

let PlaylistType = new GraphQLObjectType({
  name: 'Playlist',
  description: 'A playlist',
  fields: () => ({
    id: {
      type: new GraphQLNonNull(GraphQLID)
    },
    title: {
      type: GraphQLString
    },
    _creatorId: {
      type: GraphQLString
    },
    songs: {
      type: new GraphQLList(SongType)
    },
    type: {
      type: new GraphQLNonNull(GraphQLString)
    }
  }),

  interfaces: [nodeInterface]
});

let AddSongToPlaylistMutation = mutationWithClientMutationId({
  name: 'AddSong',
  inputFields: {
    id: {type: new GraphQLNonNull(GraphQLID) },
    youtubeLink: { type: new GraphQLNonNull(GraphQLString) }
  },
  outputFields: {
    playlist: {
      type: PlaylistType,
      resolve: ({id}) => {
        return Playlist.getPlaylistById(id);
      }
    }
  },

  mutateAndGetPayload: Playlist.addSong
});