Graphql:必须提供查询字符串

我有一个简单的express graphql服务器:

const schema = require('./schema');
const express = require('express');
const graphqlHTTP = require('express-graphql');
const cors = require('cors')
const bodyParser = require('body-parser');


const app = express();
app.use(cors());
app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());

app.use('/graphql', graphqlHTTP(req => {
  return ({
  schema,
  pretty: true,
  })
}));

const server = app.listen(9000, err => {
  if (err) { return err; }
  console.log(`GraphQL server running on http://localhost:${9000}/graphql`);
});

有什么帮助吗?

(请不要将其作为副本关闭,因为其他帖子没有提供足够的信息,说明用户是如何解决的)