Foreign Key to Unique Field Update Failure

I am getting an error: 'Notice: Undefined index: sysname in ./libraries/Doctrine/ORM/Persisters/BasicEntityPersister.php on line 434' when I try to flush a change to a property that references a unique field on another object.

From poking around in the _prepareUpdateData function, it seems that it only allows you to use identifier fields:

$newValId = $uow->getEntityIdentifier($newVal);

..

$result[$owningTable][$sourceColumn] = $newValId[$targetClass->fieldNames[$targetColumn]];

I'll see if I can get a test case for this set up.