Create objects with references Node Js and MoongoDB

Hello, I have the following data models in mongo db

var mongoose = require("mongoose");
var accountSchema = new mongoose.Schema({
    name: String
});

module.exports = mongoose.model("Account",accountSchema);

A second model

var mongoose = require("mongoose");

var bundleSchema = new mongoose.Schema({
    bundle: String,
    ban: String,
    country: String
});

module.exports = mongoose.model("Bundle",bundleSchema);

and a third model that relates both models

var mongoose = require("mongoose");

var serviceSchema = new mongoose.Schema({    
    service: String,

    account:{         
            type: mongoose.Schema.ObjectId,
            ref: "Account"
        },     
    bundle: {        
                type: mongoose.Schema.Types.ObjectId,
                ref: "Bundle"            
        }
});
module.exports = mongoose.model("Service",serviceSchema);

The problem I have is when creating the object with references it tells me that the following error

  

CastError: Cast to ObjectID failed for value "{account:   5bf32c6e43ff0338c69f20f} "at path" account "

This is the method I try to implement to create the object

router.post("/",function(req,res){     
    var serv = req.body.service;
    var accountName = {name:req.body.accName};
    var bundleName = {bundle:req.body.bdlNumber};

    Account.findOne(accountName,function(err,fAccount){
        if(err){console.log(err)}
        else{

            console.log(fAccount._id);
            var acc = {account:fAccount._id};

           Bundle.findOne(bundleName,function(err,fBundle){
            if(err){console.log(err);}
            else{
                console.log(fBundle._id);
                var bld = {id:fBundle._id };

                var newService = {service:serv,account:acc,bundle:bld}

                Service.create(newService,function(err,newlyCreated){
                    if(err){console.log(err);}
                    else{
                    console.log(fBundle.telco);
                    res.redirect("/services/")
                }})

            }
           })     
        }
    })
})

How do I create the object to later popularize the data of the other objects?

Thanks