GraphQL: Error: Schema query must be Object Type but got: [object Object]

I'm currently trying to inherit schemas for a rootQuery in order to get more modularity. The setup currently looks as follows:

invoiceSchema.js

import {
    GraphQLObjectType,
    GraphQLInt,
} from 'graphql';
export default new GraphQLObjectType({
    name: 'Invoice',
    description: 'A monthly billing invoice for an organization',
    fields: () => ({
        amountDue: {
            type: GraphQLInt,
            description: 'The amount the card will be charged (total + startingBalance with a min value of 0)'
        },
    })
});

rootQuery.js

import {
    GraphQLObjectType,
    GraphQLString,
    GraphQLInt,
    GraphQLList,
    GraphQLID
} from 'graphql';
import Invoice from './invoiceSchema';

export default {
    Invoice: {
        type: Invoice,
        resolve(parentValue, args){
            return 'Hello world';
        } 
    }
};  

schema.js

import query from './rootQuery';
import {GraphQLSchema} from 'graphql';

export default new GraphQLSchema({query});

When trying to do the following error and hoped for some help and insight, as what I'm exporting in invoiceSchema.js is clearly an ObjectType and not an object Object.

C:\project\node_modules\graphql\jsutils\invariant.js:19
    throw new Error(message);
    ^

Error: Schema query must be Object Type but got: [object Object].
    at invariant (C:\project\node_modules\graphql\jsutils\invariant.js:19:11)
    at new GraphQLSchema (C:\project\node_modules\graphql\type\schema.js:72:88)
    at Object.<anonymous> (C:/project/api/schema/schema.js:6:16)
    at Module._compile (module.js:573:30)
    at loader (C:\project\node_modules\babel-register\lib\node.js:144:5)
    at Object.require.extensions.(anonymous function) [as .js] (C:\project\node_modules\babel-register\lib\node.js:154:7)
    at Module.load (module.js:507:32)
    at tryModuleLoad (module.js:470:12)
    at Function.Module._load (module.js:462:3)
    at Module.require (module.js:517:17)
[nodemon] app crashed - waiting for file changes before starting...  

Actually got the idea from here and I'm wondering why it doesn't work...