additionalSignUpFields checkbox appearing as input type=text

Have a feeling I’m missing something simple here but when I ask for a checkbox as an additionalSignUpField, I get an input type=text with the word true in it. My (Angular 4, typescript) code is here:

lock = new Auth0Lock('abc123', '',{
        auth: {
            redirectUrl: location.origin + '/login',
            responseType: 'token',
        additionalSignUpFields: {
            type: "checkbox",
            name: "newsletter",
            prefill: "true",
            placeholder: "Keep me informed (we will not abuse this and you can unsubscribe at any time)"

Any ideas?

Sorted by upgrading to Lock 10.21.1