get an error querying config db from mongos if shard 0 is down sometimes. seems to happen once per mongo shell invocation then subsequent calls work???

$ mongo
MongoDB shell version: 2.3.0-pre-
connecting to: test
type "help" for help
mongos> use config
switched to db config
mongos> show collections
Thu Oct 04 12:23:31 uncaught exception: error:

{ "$err" : "ReplicaSetMonitor no master found for set: s0", "code" : 10009 }

mongos> show collections
// works at this point...