No error message displayed for custom radio field

-- HikaShop version -- : 2.2.0
-- Joomla version -- : 3.1.5
-- PHP version -- : 5.1

I've created a custom radio field in user tab. I've created 2 values: "Yes" and "No". Field is mandatory.
"No" is disabled and default value is setted to "No". I've filled the Error Message with a variable that I want to translate and the tested my registration form.
If I do no click over the radio to set it to "Yes", I would expect that when I click on register button, my error message would be displayed but that's not. The page just reloads, no user is created (correctly) but no error message is displayed to inform user that he must click over "Yes".

What's wrong?

Attached you can see the screenshot of my radio button.

When I go to registration page, I fill all fields except the radio (just for testing) and when I click on register button, no error message inform me that I should fill that required field (maybe because it's already pre-filled with value "No"...?)

Tested as you suggested but still same issue. Just reload the page, no user created but no message of error displayed.

Unfortunatly I cannot use a single value because italian laws want that I use something like that with 2 values.

Thanks anyway. Hope you can find a solution for my case in future.