Duplicate registrants and blank registrants

Hello all,
I have an event created and users can register other users. However, multiple registrations per user are not allowed.

Then I add 5 event products to cart and proceed to checkout. In the registrant fields, I set registrants 1 and 2 to an empty string and registrants 3, 4 and 5 to the same username.

All registrant fields are set to use the "Other account" option in the "This registration is for: (required)" dropdown.

So I expect this entry to return an error as 2 username slots are blank and the remaining username slots contain the username. But it goes right through to the next checkout step showing the 5 registrations.