Undefined index ("first_name") error when registering additional CiviEvent participants

Running D7.15/C4.2.2, created simple CiviEvent, tried unsuccessfully to register existing AND new contacts while logged in as administrator.

Successfully added myself as admin as Participant 1 - no problem. When attempting to add another existing contact OR new contact as Participant 2, I was bounced to the same screen ("Register Participant 2 of 3") with the following errors at the top of the page:

Notice: Undefined index: first_name in CRM_Event_Form_Registration_AdditionalParticipant::formRule() (line 444 of /srv/galecia.com/infopeople.galecia.com/www/sites/all/modules/civicrm/CRM/Event/Form/Registration/AdditionalParticipant.php).
Notice: Undefined index: first_name in CRM_Event_Form_Registration_AdditionalParticipant::formRule() (line 444 of /srv/galecia.com/infopeople.galecia.com/www/sites/all/modules/civicrm/CRM/Event/Form/Registration/AdditionalParticipant.php).
Notice: Undefined index: last_name in CRM_Event_Form_Registration_AdditionalParticipant::formRule() (line 445 of /srv/galecia.com/infopeople.galecia.com/www/sites/all/modules/civicrm/CRM/Event/Form/Registration/AdditionalParticipant.php).
Notice: Undefined index: last_name in CRM_Event_Form_Registration_AdditionalParticipant::formRule() (line 445 of /srv/galecia.com/infopeople.galecia.com/www/sites/all/modules/civicrm/CRM/Event/Form/Registration/AdditionalParticipant.php).

I didn't see this specific error in any recent Issues or Google results.