Why Mongoose is responding null field when it exists in DB

I don't know if this a mongoose bug or I'm doing something wrong, my problem is:

I have some documents in the mongoDB these have an attribute called address and inside it have an country which is an object id but when I make a query using mongoose this country id comes as "null":

MongoDB document:

{
   "_id" : ObjectId("54b7ff802d244c9f224c78f4"),
   "password" : "12345",
   "email" : "email@email.com",
   // ...
   "address" : {
       "country" : ObjectId("54e635cb4ef1d41d99b837e8"),
       "zipCode" : "",
       "city" : "",
       "address" : ""
   }
}

Mongoose query

Model.findOne({
   email: 'email@email.com',
   password: '12345'
}, function(err, model) { /* ... */ });

Mongoose response

{
"_id": "54b7ff802d244c9f224c78f4",
"email": "email@email.com",
"password" : "12345",
// ...
"address": {
    "country": null,
    "zipCode": "",
    "city": "",
    "address": ""
}

}

I really don't know why country is coming as null. My Mongo version is 2.6.6 and mongoose version is 3.8.21.

Any ideas?