Почему я получаю сообщение об ошибке «Необходимо предоставить строку запроса». экспресс-graphql?

var createError   = require('http-errors');  
var express       = require('express');  
var path          = require('path');  
var cookieParser  = require('cookie-parser');  
var logger        = require('morgan');  
let bodyParser    = require('body-parser');  
var cors          = require('cors');  
    
const graphqlHTTP = require('express-graphql');  
const MySchema    = require('./schema/schema');  

app.use(cors());
app.use(logger('dev'));
app.use(bodyParser.urlencoded({extended: true}));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

app.use('/graphql', graphqlHTTP({
  schema: MySchema,
  graphiql: true,
}));

const graphql = require('graphql');
const {
    GraphQLObjectType, 
    GraphQLString, 
    GraphQLSchema,
    GraphQLList
} = graphql;


//MODELS FROM MONGOOSE
const Entidad = require('../models/entidad');


//TYPES GRAPHQL
const EntidadType = new GraphQLObjectType({
    name: 'Entidad',
    fields : () => ({
        id      : {type : GraphQLString},
        nombre  : {type : GraphQLString},
        created_at : { type : GraphQLString},
        updated_at : { type : GraphQLString},
    })
});

//ROOT QUERY
const RootQuery = new GraphQLObjectType({
    name : 'RootQueryType',
    fields : {
        entidad : {
            type: EntidadType,
            args: {id:{type: GraphQLString}},
            resolve(parent, args){
                //code to get data from db
                return Entidad.findById(args.id);
            }
        }
    }
});

module.exports = new GraphQLSchema({
    query: RootQuery
});