Webform won't allow user to submit if total is zero

I have a Drupal 7 webform that enables users to register for up to two optional events. I've configured the conditional logic on the webform to support a discount code that can be entered into a field, which then sets the registration fee for one of the events to zero. If the user chooses not to register for the second event, then the total amount is zero.

When Stripe is selected as the payment processor, the form submit button does nothing, and the user is stranded having entered all their data.

Expected behaviour: The user should be able to submit the form regardless of whether the total is zero.