Error: middleware options must contain a schema using express-graphql

Question

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.