NFCNDEFTag writeNDEF writes in utf16

I am using the new nfc write capabilities and are writing successful to tags. But my payload data becomes written in utf-16. I want it to be utf8. How do I accomplish that?

This is how I create and write the NDEF:

let textPayload = NFCNDEFPayload.wellKnownTypeTextPayload(string: tmp, locale: Locale(identifier: "En"))
self.ndefMessage = NFCNDEFMessage(records: [(textPayload!)])



tag.writeNDEF(self.ndefMessage!) { (error: Error?) in
                            print(error ?? "error")
                            if error != nil {
                                session.invalidate(errorMessage: "Update tag failed. Please try again.")
                            } else {
                                session.alertMessage = "Update success!"
                                session.invalidate()                                
                            }
                        }