Duplicate key error index on embedded document in mongoose

I am trying to run this simple embedded document using mongoose:

var mongoose = require('mongoose');
var PageSchema = new mongoose.Schema({
    url:String
});
var AlbumSchema = new mongoose.Schema({
    pages:[ PageSchema ]
});

mongoose.model('Album', AlbumSchema);
var Album = mongoose.model('Album');
var album = new Album({pages:[{url:"1"}]});
album.save(function(err, a) {
    console.log(err);
});

After I run this code the second time I get this Error:

{ 
    [MongoError: E11000 duplicate key error index: doalbums.albums.$pages.id_1  dup key: { : null }]
    name: 'MongoError',
    err: 'E11000 duplicate key error index: doalbums.albums.$pages.id_1  dup key: { : null }',
    code: 11000,
    n: 0,
    connectionId: 161,
    ok: 1 
}

What am I doing wrong?