Required option not working?

When adding the star rating field in the review form, I’d like this field to be required:

[starrating* rating min:0 max:5 step:1 disable_cancel]

But I can still submit the form without choosing a number of stars. There’s no error message either. This results in (positive) client ratings with empty star rating results. That’s not very useful.

I tried different settings, different browsers, nothing seems to solve this problem.
Is this issue known? Is there a solution?

BTW, I know that there is another topic about the same issue but that one is closed and doens’t provide an answer or solution. I hope someone can help me out!

Thanks in advance.

The page I need help with: [log in to see the link]