Event Registration page allows registration even if no number (or the number 0) has been entered in any ticket quantity boxes

Scenario / How to Replicate:
The user creates an event page, using a price set that features a few 'Text / Numeric Quantity' boxes for users to specify how many of each ticket type they wish to purchase e.g. 3 student tickets, 4 standard tickets.

Problem:
Even if no number is entered into any of the quantity boxes, or the number entered is 0, the 'Continue' button still remains visible and thus allows the user to register.

Consequence:
Users can register successfully for an event without paying. This has already happened once with one of our events.

Our current workaround is some javascript/jquery in custom wordpress template applied to that page, which implements the required functionality (stopping the 'Continue' button displaying unless there is a non-zero value and no zero values in the quantity boxes).