ios – applicationUsername.toLowerCase isn’t a operate

I am attempting so as to add https://github.com/apple/app-store-server-library-swift to my server, extra particularly “Promotional Supply Signature Creation” on the backside of the web page.

After I run the code I get an error saying

TypeError: applicationUsername.toLowerCase isn't a operate at PromotionalOfferSignatureCreator.createSignature when calling:

const signatureCreator = new PromotionalOfferSignatureCreator(encodedKey, keyId, bundleId)
const signature = signatureCreator.createSignature(productId, subscriptionOfferId, signatureCreator)

Is there something I can do about it?

I’ve included the bundle as follows:
"@apple/app-store-server-library": "^1.0.1"

and I embody it as follows:

import { PromotionalOfferSignatureCreator } from '@apple/app-store-server-library'

For the operate to generate my signature