Including Protobuf returns "exports" is read only

Question

I am trying to include the following as protobuf/index.js

const protobuf = require('protobufjs')

const root = protobuf.Root.fromJSON(require('./protobuf_bundle.json'))

const Message = root.lookup('Message')
Message.MessageType = Message.nested.MessageType.values
Message.MessageType.stringValue = (id) =>
    `${Message.nested.MessageType.valuesById[id]}(${id})`

let exportableMessages =
    Object.keys(root)
        .filter((key) => /^[A-Z]/.test(key))
        .reduce((acc, key) => {
            acc[key] = root[key]
            return acc
        }, {})

// Add our stringValue enabled Message
exportableMessages['Message'] = Message

module.exports = exportableMessages

However when I include this into any other file by using

import protobuf from '../protobuf'

I get

TypeError: "exports" is read-only

I can't see anything glaringly obvious, nor can I fathom what it could be if it isn't this.