Error: Expected undefined to be a GraphQL schema.

[tykwon97]

An error continues to occur while practicing the graphql example. I don't know the cause.
I'd appreciate it if you could help me😀

const { graphql, buildSchema } = require("graphql")

const schema = buildSchema(`
  type Query {
    hello: String,
    nodejs: Int
  }
`);

const root = { 
    hello: () => 'Hello world!' ,
    nodejs: () => 20 ,
};

graphql(schema, '{ nodejs }', root).then((response) => {
  console.log(response);
});

The error message is as follows.

(node:20348) UnhandledPromiseRejectionWarning: Error: Expected undefined to be a GraphQL schema.
    at assertSchema (C:\study\JavaScript\Node\GraphQL\node_modules\graphql\type\schema.js:37:11)
    at validateSchema (C:\study\JavaScript\Node\GraphQL\node_modules\graphql\type\validate.js:34:28)
    at graphqlImpl (C:\study\JavaScript\Node\GraphQL\node_modules\graphql\graphql.js:60:64)
    at C:\study\JavaScript\Node\GraphQL\node_modules\graphql\graphql.js:23:43
    at new Promise (<anonymous>)
    at graphql (C:\study\JavaScript\Node\GraphQL\node_modules\graphql\graphql.js:23:10)
    at Object.<anonymous> (C:\study\JavaScript\Node\GraphQL\start.js:16:1)
    at Module._compile (internal/modules/cjs/loader.js:1063:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
    at Module.load (internal/modules/cjs/loader.js:928:32)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:20348) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)
(node:20348) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.