authentication for replica set fails frequently because 'getnonce' and 'authenticate' command is issued to different server

following code returns false.

import com.mongodb.Mongo

import com.mongodb.ServerAddress

val servers = new java.util.ArrayList[ServerAddress]

servers.add("192.168.10.1",27017)

servers.add("192.168.10.2",27017)

servers.add("192.168.10.3",27017)

val mongo = new Mongo(servers)

mongo.slaveOk // <= IMPORTANT!

mongo.getDB("dbname").authenticate("user","pass".toCharArray)

'getnonce' and 'authenticate' command seems to be issued to different server.
( 'getnonce' issued to 192.168.10.2, 'authenticate' issued to 192.168.10.3)

and logged following message on the server which received 'authenticate' command.

auth: bad nonce received or getnonce not called. could be a driver bug or a security attack. db:dbname

'getnonce' and 'authenticate' should be issued to same server whether 'slaveOk' or not.