How to make oplogMainRowCount appear on MongoDB

gistfile1.js

// delete oplog, crete new and empty
testReplSet:PRIMARY> use local
switched to db local
testReplSet:PRIMARY> db.oplog.rs.drop()
true
testReplSet:PRIMARY> db.createCollection("oplog.rs", {capped:1, size: 2*1024*1024, autoIndexId:false})
{ "ok" : 1 }
// run the command to get the fields
testReplSet:PRIMARY> db.getReplicationInfo()
{
        "logSizeMB" : 2,
        "usedMB" : 0,
        "errmsg" : "objects not found in local.oplog.$main -- is this a new and empty db instance?",
        "oplogMainRowCount" : 0
}
// insert anything, to populate the oplog
testReplSet:PRIMARY> use test
switched to db test
testReplSet:PRIMARY> db.foo.insert({"a" : 1})
// and now the extra fields are gone
testReplSet:PRIMARY> db.getReplicationInfo()
{
        "logSizeMB" : 2,
        "usedMB" : 0.01,
        "timeDiff" : 0,
        "timeDiffHours" : 0,
        "tFirst" : "Mon May 06 2013 14:17:09 GMT+0100 (IST)",
        "tLast" : "Mon May 06 2013 14:17:09 GMT+0100 (IST)",
        "now" : "Mon May 06 2013 14:17:11 GMT+0100 (IST)"
}