Registering multiple people for event fails when first person is free, and later people have a cost

This issue can be reproduced in the CiviCRM sandbox. To reproduce it:

1) Go to http://drupal.demo.civicrm.org/civicrm/event/register?reset=1&id=9

2) When asked "How many people are you registering?" Choose 2 or more

3) When choosing the price for the first/main person, choose "Adult - complimentary - € 0,00"

4) Click "Continue"

5) For this person choose "Child - paid guest - € 25,00 " and choose "Continue"

6) You will see the error message "Please correct the following errors in the form fields below:
Your payement information looks incomplete. Please go back to the main registration page, to complete payment information."

This error happens no matter if someone chooses "pay later" or pays online.