On one of our environments, we have a replica set with only one member. Issuing a "count" command on a collection, against this member (which is the primary, of course) does not return a response.

We traced the issue to the mongodb-core/topologies/replset.js file, line 1067 - the driver stashes the command for future execution.

This only occurs if our connection string features a replicaSet parameter, like so: "host:27017/DatabaseName?replicaSet=replicaSetName", otherwise it is probably not considered a replica set, haven't checked exactly why.