Navigator credentials creation method returning an exception

Trying to run a demo for the webautn spec (https://www.w3.org/TR/webauthn/) available (https://github.com/molekilla/webauthn-demo-fork) under Firefox Nightly.

getMakeCredentialsChallenge({
        username,
        name
    })
    .then((response) => {
        console.log(response);

        let publicKey = preformatMakeCredReq(response);

        console.log(publicKey);

        console.log(publicKey.challenge)

        return navigator.credentials.create({publicKey})
    })

Whenever the execution reaches the return statement, the Promise will stay pending for a few seconds and eventually rejects, logging either [Exception... "Abort" nsresult: "0x80004004 (NS_ERROR_ABORT)" location: "<unknown>" data: no] or UnknownError: The operation failed for an unknown transient reason. Both objects seem fine. Any idea about the reason why is it not resolving?