Search the _id inside an array with Mongoose

This is the code:

usermodel.findOne({"imagen._id": req.params.id }, {imagen: 1}, function (err, imagen){

    if(err) throw err;
    console.log(req.params.id + '\n');
    console.log(imagen);

    res.send(imagen);

});

And this is the Schema:

var userschema = new mongoose.Schema({

  user: String,
  imagen: [{ 

              title: String,
              name: String,
              author: String,


           }]

});

I'm trying to get an output like this:

{  title: 'DQ monstes',
   name: 'fooname.png',
   author: 'fooauthor',
}

But I get this:

{ _id: 50f41ccff405ef73c4000006,
imagen: 
 [ { title: 'Pokemon',
   name: 'random.png',
   author: 'fooauthor',
},
  { title: 'DQ monstes',
    name: 'fooname.png',
    author: 'fooauthor',
}

The _id at the beginning is the req.params._id that I'm looking for at the usermodel.findOne. And I receive this error too:

/Users/rodrie/Documents/node/node_modules/mongoose/lib/utils.js:434
     throw err;
           ^
TypeError: Cannot read property '_id' of null