Wordpress Contact Form 7 radio not required

I have a form with few fields along with a radio button. This radio button is an optional field. But I am not able to submit form if I do not select any option from the radio button.

<div class="input-field-outer">
    [text* your-name class:name placeholder "Name or business name*"]
</div>
<div class="input-field-outer">
    [email* email placeholder "Email*"]
</div>
<div class="input-field-outer">
    [text* contact-number id:telno class:hs-phone-number placeholder "Contact number*"]
    <span id="errmsg"></span>
</div>
<div class="input-field-outer">
    <div class="radio-outer">
        [radio customer_type "New Customer" "Old Customer"]
    </div>
</div>

It will work when I make any of them selected default:

[radio customer_type default:1 "New Customer" "Old Customer"]

Any ideas about using radio button with no items default.