The test-only command 'emptycapped' drops and recreates all indexes. Because of this, it is possible for dbHash to pick a read timestamp where the new _id index is not visible yet. Instead of returning a 'no _id index' error, dbHash should return a SnapshotUnavailable error, which is already an expected and routine error code. 

This problem can be fixed by having 'emptycapped' set the collection's minimumVisibleSnapshot.