must have a selection of subfields. Did you mean \"createEvent { ... }\"?", [graphql]

Question

I am learning graphql so please have patience with me.

My app.js looks like this:

const express = require('express');
const bodyParser = require('body-parser');
const graphqlHttp = require('express-graphql');
const { buildSchema } = require('graphql');
const mongoose = require('mongoose');
const Event = require("./models/event");

const app = express();

app.use(bodyParser.json());

app.use('/graphql', graphqlHttp({

    schema: buildSchema(`
        type Event{
            _id: ID!
            title: String!
            description: String!
            price: Float!
            date: String!
        }

        input EventInput{
            title: String!
            description: String!
            price: Float!
            date: String!
        }

        type RootQuery{
            events: [Event!]!
        }

        type RootMutation {
            createEvent(eventInput: EventInput): Event
        }

        schema {
            query: RootQuery
            mutation: RootMutation
        }
    `),
    rootValue: {
        events: () => {
            return events;
        },
        createEvent: args => {
            const event = new Event({
                title: args.eventInput.title,
                description: args.eventInput.description,
                price: +args.eventInput.price,
                date: new Date(args.eventInput.date)
            });
            return event
                .save()
                .then(result => {
                    console.log(result);
                    return {...result._doc};
                })
                .catch(err => {
                    console.log(err);
                    throw err;
                });            
        }        
    },
    graphiql: true
}));


mongoose.connect(`mongodb://${process.env.MONGO_USER}:${process.env.MONGO_PASSWORD}@localhost:27017/graphqldb`, {useNewUrlParser: true});

var db = mongoose.connection;

db.on('error', console.error.bind(console, 'connection errror:'));
db.once('open', function(){
    console.log('Connection open');
    app.listen(5000);
});

and my event.js looks like this:

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const eventSchema = new Schema({
    title: {
        type: String,
        required: true
    },
    description: {
        type: String,
        required: true
    },
    price: {
        type: Number,
        required: true
    },
    date: {
        type: Date,
        required: true
    }
});

module.exports = mongoose.model('Event', eventSchema);

when I tried to very it, by accessing the URL: http://localhost:5000/graphql

mutation {
  createEvent(eventInput: {title: "title", description: "description", price: 9.99, date: "2019-05-15T08:04:26.461Z"})
}

I encounter an error related to subfields:

{ "errors": [ { "message": "Field \"createEvent\" of type \"Event\" must have a selection of subfields. Did you mean \"createEvent { ... }\"?", "locations": [ { "line": 3, "column": 3 }

Please let me know what I am missing. As I understand my field does not contain any subfields.

Thank you in advance.

UPDATE:

This is not a duplicate from the questions posted. As you can see in my mutation, I supplied all the required fields but still I recieved this error.

The error happened only when I tried to use mongoose but before it is working as expected.