Cannot reset target specified by syncFrom()

I could specify the sync target by using rs.syncFrom("[host]:[port]").
And I also could overwrite the sync target.
But I could not reset the target !

RS:SECONDARY> rs.syncFrom(null)

{

        "syncFromRequested" : "",

        "ok" : 0,

        "errmsg" : "could not find member in replica set"

}

Is there the way to reset the sync target ?
Or I have to restart the mongod process ?
(sync target is merely saved on memory in process)