return only one item in collection mongoose

I have method getFollowing to get list of following user

UserSchema.statics.getFollowing = function(data, callback) {
    console.log("data: ", data);
    this.findOne({
        name : data.name
    }, function(err, docs) {
        if (err)
            callback("error " + err)
        else if (docs) {
            console.log("XXXX", docs.following_count)
            mongoose.model('User').findOne({
                                     _id : docs._id
                                }).populate('following', 'name screen_name avatar _id').exec(function(err, res) {
                                    if (err)
                                        callback(err);
                                    // console.log('The following is %s', res)                          

                                });         

            callback(docs.following);
        }
        else callback('{"error": "user-not-found"}');
    });
}

and I want to callback send to client docs.following list only I dont know when I console.log(docs.following) it print but when I callback(docs.following) it return err.

I define UserSchema like that

var UserSchema = new Schema({
created_at : { type: Date, default: Date.now }
, name : String
,   hashedPass : String
,   salt: String
,   email : Email
,   avatar : {type: String, default: "images/default_profile.png" }
,   statuses_count : Number
,   screen_name : String
,   location : String
,   about : String
,   followers: [{ type: ObjectId, ref: 'User' }]
,   following: [{ type: ObjectId, ref: 'User' }]
,   followers_count : Number
,   following_count : Number
});