Error: cannot call next() after end()

leveldown-throw-error.js

var leveldown = require("leveldown")
  , db = leveldown("./db")
  , async = require("async")
  , count = 0
  , iterators = []
  , mythrow = function(err) {
      console.log(err)
    }

async.series([
  function(cb) {
    db.open(cb)
  },
  function(cb) {
    async.whilst(
      function() { return count < 100 },
      function(callback) {
        count++;
        db.put("a" + count, "b" + count, callback);
      },
      cb
    )
  },
  function(cb) {
    iterators.push(db.iterator())

    db.close(cb)
  },
  function(cb) {

    iterators.forEach(function(i) {
      i.next(mythrow)
    })

    cb()
  },
  function(cb) {
    console.log("done!")
    cb()
  }
])