Can't pay for event with more then one participant

I'm running against the 6.6 (unreleased) branch.

When I register for a paid event, with more then one participant, when I click the "Review your registration button" the button greys out, but nothing else happens.

Judging from the console output, it looks like getTotalAmount() returns null (as programmed) but whatever is calling getTotalAmount() expects a number (or float).