I have recently setup a sharded cluster, but the shards are actually single nodes. The problem I'm having is that when I query the `mongos` instance, reusing some code I know works against a single node I get:

Exception in thread "main" com.mongodb.CommandResult$CommandFailure: command failed [mapreduce]:

{ "serverUsed" : "project11.cs.st-andrews.ac.uk/138.251.214.135:27017" , "ok" : 0.0 , "errmsg" : "unknown m/r field for sharding: $readPreference"}

Note: I was performing an aggregate using the new aggregate framework.