Why my mongoose objectId request return an empty array?

I past my day to find the answer, in vain.

this is my mongoose schema:

var schema = new mongoose.Schema({
    value: {type: String},
    title: {type: String},
    date: { type: Date, default: Date.now },
    packId: {type: mongoose.Schema.Types.ObjectId, ref:'Pack'}
});

this is my request:

( Where packId is a ObjectId )

var getElements = function(packId, callback ){
    Card.find(
        {packId: packId},
        function(err,els){
            if(err){
                console.log(err);
            }else{
                console.log(els);
            }
        }
    );
};

**And this is a table element **

{
    "title" : "test1",
    "value" : "test1",
    "packId" : ObjectId("54f9ebaae312727c45b2a80e"),
    "_id" : ObjectId("54f9ebaae312727c45b2a820"),
    "date" : ISODate("2015-03-06T18:02:18.544Z"),
    "__v" : 0
}

I don't understand why the console.log(els) return [], because there is elements in table. I've tried whith {type: Sting} but nothing at all.

Thanks