WriteResult._id is undefined in case of upsert

The WriteResult from an upsert displays in the shell like so:

res = db.c.update(..., ..., {upsert: true})

printjson(res)

{

	"nMatched" : 0,

	"nUpserted" : 1,

	"nModified" : 0,

	"_id" : ObjectId("5ca4b9271da31f1c4f6cc755")

}

However attempting to access the _id returns undefined:

printjson(res._id)

undefined

The returned _id can be accessed as res.getUpsertedId()._id, but the value printed for res and the documentation indicate that it should be available as res._id.