Incomplete error message when TOS are not checked during registration

[generalecluster]

Hello, I've a problem with Virtuemart 3.2.4 and Joomla 3.8.1 when a new user registers on cart checkout.
I've set a "tos" field as required and set it to be shown on the account management form.
If the user leaves the checkbox unchecked, the javascript validator shows a popup with the error message "Required field is missing".
An error message area with red background at the top of the page is shown, but it's empty. I'd expect it to contain a text like "Invalid field: Terms of service" instead.
I've tried both my custom template and protostar, with the same result.
How may I do?
Thank you.