Exception: "The given document has no identity." when findOne query returns none from a MongoGridFs

When running a query against a GridFS that should return no results, I expect the return value to be null (as with normal documents) instead an InvalidArgumentException is thrown, with the message "The given document has no identity".

It appears the empty result is trying to be registered as a managed document with the UOW, because MongoCollection does not return null in this case.