[Bug] Document.prototype.populate() options.retainNullValues not working.

[bongjinpark1]

nodejs version : 10.16.3
mongooser version : 5.7.7

teams.js

const mongoose = require('mongoose')
const fields = {
  troops: [{ type: Number, ref: 'Card' }]
}

const options = {
  versionKey: false,
  timestamps: true
}

const schema = new mongoose.Schema(fields, options)

const Team = mongoose.model('Team', schema)

module.exports = Team

cards.js

const mongoose = require('mongoose')

const fields = {
  _id: { type: Number },
  name: { type: String, unique: true },
  entityType: { type: String }
}

const options = {
  discriminatorKey: 'entityType'
}

const schema = new mongoose.Schema(fields, options)

const Card = mongoose.model('Card', schema)

module.exports = Card

index.js

    const Team = require('path of teams.js')
    const team = new Team({
      troops: [1, 2, 3, 4]
    })

    await team.save()

    await team
      .populate([
        {
          path: 'troops',
          select: 'name',
          retainNullValues: true, // not working
          options: {
            retainNullValues: true // not working too.
          }
        }
      ])
      .execPopulate()

    console.log(team)

/*
expected:
{
troops: [null, {name: 'name of 2'}, {name: 'name of 3'}, {name: 'name of 4'}]
}

actual:
{
troops: [{name: 'name of 2'}, {name: 'name of 3'}, {name: 'name of 4'}]
}
*/

How can i get populated array include null value? any ideas?

thanks.