We allow upgrades after unclean shutdown. After an unclean shutdown, it's possible we'd have a non-null appliedThrough and a null stableTimestamp in wiredTiger (so it wouldn't be boost::none). We should add an "|| stableTimestamp->isNull()" check to this invariant: https://github.com/mongodb/mongo/blob/d12afa4fdda9c6b113e7be3b4d67d757b07b50b5/src/mongo/db/repl/replication_recovery.cpp#L215-L219