If you only have one member in the replica set and try to do a slaveOk query, the mongos will look for a slave to send the query to, and when it cant find one it will call isMaster and replSetGetStatus to try and update its view of the state of the replSet to see if a secondary came online. However as long as there is only one node in the set, there is no chance of that happening. It should probably either just return the connection to the primary and log a warning, or it should throw an error and not process the query.