We have a check in DocumentSourceCursor to ensure that the current node in a replica set can serve reads. Due to the fact that in SBE we use an internal lock policy, we changed this check to only perform it when the lock policy is external, since in DocumentSourceCursor we acquire a collection lock ourselves. That means that in SBE we do not perform this check any longer, so we have to figure out who is going to perform this check in SBE and implement it.