Start a 2.6.4 single-member RS: > db.serverBuildInfo().version 2.6.4 > rs.initiate({ _id: 'rs' , members:[{_id: 0, host: 'localhost:27017' }], settings: {getLastErrorDefaults: {w:0}} }) { "info" : "Config now saved locally. Should come online in about a minute." , "ok" : 1 } rs:OTHER> rs.status().members[0].stateStr PRIMARY Now start a member running the latest code from master (I have 4bea2a90) with the option "-vvvv". Try to add it from the shell connected to the 2.6.4 primary: rs:PRIMARY> rs.add( 'localhost:27018' ) { "ok" : 1 } The primary thinks the member was added. However: rs:PRIMARY> rs.status() { "set" : "rs" , "date" : ISODate( "2014-10-14T16:27:05Z" ), "myState" : 1, "members" : [ { "_id" : 0, "name" : "localhost:27017" , "health" : 1, "state" : 1, "stateStr" : "PRIMARY" , "uptime" : 436, "optime" : Timestamp(1413303922, 1), "optimeDate" : ISODate( "2014-10-14T16:25:22Z" ), "electionTime" : Timestamp(1413303652, 2), "electionDate" : ISODate( "2014-10-14T16:20:52Z" ), "self" : true }, { "_id" : 1, "name" : "localhost:27018" , "health" : 1, "state" : 6, "stateStr" : "UNKNOWN" , "uptime" : 103, "optime" : Timestamp(0, 0), "optimeDate" : ISODate( "1970-01-01T00:00:00Z" ), "lastHeartbeat" : ISODate( "2014-10-14T16:27:04Z" ), "lastHeartbeatRecv" : ISODate( "1970-01-01T00:00:00Z" ), "pingMs" : 0, "lastHeartbeatMessage" : "still initializing" } ], "ok" : 1 } The member we tried to add logs "User Assertion: 17505:replSet illegal config: getLastErrorDefaults w:0": 2014-10-14T12:25:22.093-0400 I NETWORK [initandlisten] connection accepted from 127.0.0.1:62519 #1 (1 connection now open) 2014-10-14T12:25:22.093-0400 D COMMANDS [conn1] run command admin.$cmd { _isSelf: 1 } 2014-10-14T12:25:22.093-0400 I QUERY [conn1] command admin.$cmd command: _isSelf { _isSelf: 1 } ntoreturn:1 keyUpdates:0 numYields:0 reslen:53 0ms 2014-10-14T12:25:22.093-0400 D NETWORK [conn1] Socket recv() conn closed? 127.0.0.1:62519 2014-10-14T12:25:22.093-0400 D NETWORK [conn1] SocketException: remote: 127.0.0.1:62519 error: 9001 socket exception [CLOSED] server [127.0.0.1:62519] 2014-10-14T12:25:22.093-0400 I NETWORK [conn1] end connection 127.0.0.1:62519 (0 connections now open) 2014-10-14T12:25:22.093-0400 I NETWORK [initandlisten] connection accepted from 127.0.0.1:62520 #2 (2 connections now open) 2014-10-14T12:25:22.093-0400 D COMMANDS [conn2] run command admin.$cmd { replSetHeartbeat: "rs" , v: -1, pv: 1, checkEmpty: false , from: "", fromId: 0 } 2014-10-14T12:25:22.093-0400 D COMMANDS [conn2] command: { replSetHeartbeat: "rs" , v: -1, pv: 1, checkEmpty: false , from: "", fromId: 0 } 2014-10-14T12:25:22.093-0400 I QUERY [conn2] command admin.$cmd command: replSetHeartbeat { replSetHeartbeat: "rs" , v: -1, pv: 1, checkEmpty: false , from: "", fromId: 0 } ntoreturn:1 keyUpdates:0 numYields:0 reslen:72 0ms 2014-10-14T12:25:22.094-0400 D COMMANDS [conn2] run command admin.$cmd { replSetHeartbeat: "rs" , v: 2, pv: 1, checkEmpty: false , from: "localhost:27017" , fromId: 0 } 2014-10-14T12:25:22.094-0400 D COMMANDS [conn2] command: { replSetHeartbeat: "rs" , v: 2, pv: 1, checkEmpty: false , from: "localhost:27017" , fromId: 0 } 2014-10-14T12:25:22.094-0400 I QUERY [conn2] command admin.$cmd command: replSetHeartbeat { replSetHeartbeat: "rs" , v: 2, pv: 1, checkEmpty: false , from: "localhost:27017" , fromId: 0 } ntoreturn:1 keyUpdates:0 numYields:0 reslen:72 0ms 2014-10-14T12:25:22.408-0400 D QUERY [rsStart] Running query: query: {} sort: {} projection: {} skip: 0 limit: 1 2014-10-14T12:25:22.408-0400 D QUERY [rsStart] Collection local.system.replset does not exist. Using EOF stage: query: {} sort: {} projection: {} skip: 0 limit: 1 2014-10-14T12:25:22.408-0400 I QUERY [rsStart] query local.system.replset planSummary: EOF ntoreturn:1 ntoskip:0 nscanned:0 nscannedObjects:0 keyUpdates:0 numYields:0 nreturned:0 reslen:20 0ms 2014-10-14T12:25:22.408-0400 D REPLSETS [rsStart] ReplSetConfig load localhost:27017 2014-10-14T12:25:22.409-0400 D COMMANDS [ConnectBG] BackgroundJob starting: ConnectBG 2014-10-14T12:25:22.409-0400 D NETWORK [rsStart] connected to server localhost:27017 (127.0.0.1) 2014-10-14T12:25:22.409-0400 I REPLSETS [rsStart] trying to contact localhost:27017 2014-10-14T12:25:22.409-0400 D COMMANDS [ConnectBG] BackgroundJob starting: ConnectBG 2014-10-14T12:25:22.409-0400 D NETWORK [rsStart] connected to server localhost:27017 (127.0.0.1) 2014-10-14T12:25:22.410-0400 D - [rsStart] User Assertion: 17505:replSet illegal config: getLastErrorDefaults w:0 2014-10-14T12:25:22.410-0400 D REPLSETS [rsStart] replSet exception trying to load config from discovered seed localhost:27017 2014-10-14T12:25:22.410-0400 I REPLSETS [rsStart] replSet can't get local.system.replset config from self or any seed (EMPTYCONFIG) 2014-10-14T12:25:22.410-0400 D REPLSETS [rsStart] replSet info no seed hosts were specified on the --replSet command line