"Call to a member function toArray() on a non-object" in PersistentCollection::takeSnapshot() during LEFT JOIN

I am getting the fatal error "Call to a member function toArray() on a non-object" which is triggered in Doctrine/ORM/PersistentCollection.php on line 239 by $this->snapshot = $this->coll->toArray();

{{
SELECT i, c FROM Page i
LEFT JOIN i.children c
WHERE i.parent_id IS NULL
AND i.site = ?1
ORDER BY i.id ASC
}}

The association is self-referential on the column parent_id. Removing "LEFT" from the query makes is run successfully (but ofc not in the way I need it). Making it only select 'i' instead of 'i, c' also solves it. But I don't think it's wrong in it's current form is it?

The Entity (slightly truncated to remove personal/local info) is at http://scrp.at/bvl