User registration form with checkbox field
Just in case anyone else has this issue i got it working with these modifications.

I think the hidden field was required for the possibility of null values.

&lt;input type="hidden" name="fields[myCustomField]" value="no"&gt;


It also required the extra [] brackets after the field name in the checkbox option (as checkbox fields are an array).

name="fields[myCustomField][]"


Here's my code thats working:
    

            &lt;input type="hidden" name="fields[termsAgreement]" value="no"&gt;
            &lt;input type="checkbox" id="termsAgreement" name="fields[termsAgreement][]" value="yes"&gt;

             I agree to terms and conditions above
            &lt;/label&gt;
            {{ account is defined and account ? errorList(account.getErrors('termsAgreement')) }}