Error: middleware options must contain a schema using GraphiQL

For some reason, when using the schema below, I get the error "GraphQL middleware options must contain a schema." when trying to load the GraphiQL page, and an error status 500 when making requests from my react app. Here is my code:

server.js

const express = require('express');const { graphqlHTTP } = require('express-graphql');const clientSchema = require('./schemas/clientSchema');]const app = express();const cors = require('cors');app.use(cors());app.use('/graphql', graphqlHTTP({    clientSchema,     graphiql: true}));module.exports = app;

clientSchema.js

const graphql = require('graphql');const account = require('../queries/account');const {    GraphQLObjectType,    GraphQLString,    GraphQLSchema,    GraphQLID,    GraphQLInt,    GraphQLFloat,    GraphQLList,    GraphQLNonNull,    GraphQLBoolean} = graphql;const AccountType = new GraphQLObjectType({    name: 'Account',    fields: () => ({        id: { type: GraphQLID },        name: { type: GraphQLString },        email: { type: GraphQLString },        phone: { type: GraphQLString },        password: { type: GraphQLString }    })})const Mutation = new GraphQLObjectType({    name: 'Mutation',    fields: {        createAccount: {            type: AccountType,            args: {                name: { type: new GraphQLNonNull(GraphQLString) },                email: { type: new GraphQLNonNull(GraphQLString) },                password: { type: new GraphQLNonNull(GraphQLString)}            },            async resolve(parent, args) {                return await account.createAccount({                        name: args.name,                        email: args.email,                         password: args.password                       });            }        }    }})module.exports = new GraphQLSchema({    mutation: Mutation})

Any help would be greatly appreciated. Thank you.