JOIN fails with inherited entities

"Entity\User\AbstractUser" is a base entity for user records, "Entity\User\Person" extends AbstractUser (Single Table inheritance in this case).
The "Setting" property is a OneToOne relation with another entity.

Now when I call this DQL:
SELECT u, t FROM Entity\User\AbstractUser u LEFT JOIN u.Setting t

The ObjectHydrator fails in line 276 ("if ( ! $relation->isOneToOne()) {"...) with "PHP Fatal error: Call to a member function isOneToOne() on a non-object in .../lib/Doctrine/ORM/Internal/Hydration/ObjectHydrator.php".

It works when using the sub class for the query:
SELECT u, t FROM Entity\User\Person u LEFT JOIN u.Setting t