We are vulnerable to reach to this Invariant following this scenario:

1- A follower node called for a dry run election due to catchup takeover and sent primaryIndex to voteRequester to start scatter_and_gather_runner to send voteRequests to all voters.

2- All Requests has been scheduled  with processResponse as callbacks 

3- Primary node's response is the last response to be processed, which indicates that _callbacks list is empty.

4- The request to the Primary Node failed so we end-up with failed response and we return leaving _primaryVote unchanged as  PrimaryVote::Pending.

5- So the call to hasReceivedSufficientResponses will return false causing the Invariant to fire.

Ask to add more diagnostic information: 
Can we add primaryIndex to that log line to log what is the last known primaryIndex for this node while running for the dry-run catchup takeover.