I've been trying to change the owner of a .us domain, and it looks like the process is the same as described here, but then I get a strange error about "The 'Your security answer' field is mandatory" (there is no security question or answer box anywhere).

Is there an extra step I've missed somewhere?

Same question here, but then with a .com address.

In order to change the name of the owner of the domain name to a third party, you will need to proceed with an ownership change. This can be done online directly from your domain name's control panel by clicking on the "Change Owner" link that you see in the Contact section on the bottom left-hand corner of the page.

To see a detailed example of how to change the owner of a domain name at Gandi, please visit the following page of our website at: http://wiki.gandi.net/en/domains/management/change-domain-owner1

Note that you will need to be logged in as the domain's owner or administrative contact in order to start an ownership change.

As a reminder, the Gandi Wiki and Gandi Groups are not for official support from Gandi. They are for peer-to-peer customer support. If you have any specific questions, please send in a Support Ticket: http://www.gandi.net/support/contact/