Meteor.users collection search with EasySearch

I’m trying to search Meteor.users collection document using EasySearch package but strange not returning any user, I have implemented the search for some other collections & it’s working but not working while implemented in Meteor.users, here is my implementation

in api

import { Meteor } from 'meteor/meteor';
import { EasySearch } from 'meteor/easy:search';

export const UsersIndex = new EasySearch.Index({
    collection: Meteor.users,
    fields: ['username', 'createdAt'],
    engine: new EasySearch.MongoDB()

in the client

import { UsersIndex } from './../../api/users';

Tracker.autorun(() => {
    let usersSearch ='');
    console.log('Users', usersSearch.fetch());
    console.log('Users Count', usersSearch.count());

the output is

Users []
Users Count 0

what is the issue actually?