Adding relationship with multi-valued custom field fails

When adding a relationship that has a custom field set that contains a multi-valued field, such as a checkbox, the throbber spins until the page is refreshed and the error "unknown relationship create error A fatal error was triggered: Cannot recognize for" is displayed.

I traced the problem to a block of code that was added for CRM-15829 in CRM/Contact/BAO/Relationship.php. The failure is on line 1481 (for v4.6.9). I have verified this also fails on the demo site.