TypeError:执行回调时，回调不是函数

当咖啡脚本中的流触发end事件时，我正在尝试运行回调。在尝试运行我的代码时，我收到以下错误：

C:'Users'Gum-Joe'Documents'Projects'retisci'lib'downloader.js:45
    callback();
    ^
TypeError: callback is not a function
  at Request.<anonymous> (C:'Users'Gum-Joe'Documents'Projects'retisci'lib'downloader.js:45:5)
  at emitOne (events.js:101:20)
  at Request.emit (events.js:188:7)
  at IncomingMessage.<anonymous> (C:'Users'Gum-Joe'Documents'Projects'retisci'node_modules'request'request.js:988:12)
  at emitNone (events.js:91:20)
  at IncomingMessage.emit (events.js:185:7)
  at endReadableNT (_stream_readable.js:926:12)
  at _combinedTickCallback (internal/process/next_tick.js:74:11)
  at process._tickCallback (internal/process/next_tick.js:98:9)

注意：修改输出以保护我的全名。

这是我的代码：

###
# Downloader
# @param url {String} url to GET
# @param save {String} Save file
# @param options {Object} Options
# @param callback {Function} Callback
###
downloader.get = (url, save, options, logger_options, callback) ->
  log = require('single-line-log').stdout
  # Download
  # Logger
  logger = new Logger('retis', logger_options)
  # File stream
  file_stream = fs.createWriteStream(save)
  log("#{"[retis INFO]".green} Downloading #{url}...0% at 0 kb/sec...'n")
  progress(request(url))
    .on('progress', (state) ->
      percent = "#{Math.floor(state.percentage * 100)}% [#{Math.round(state.size.transferred / 1024)} kb of #{Math.round(state.size.total / 1024)} kb]"
      log("#{"[retis INFO]".green} Downloading #{url}...#{percent} at #{Math.round(state.speed / 1024)} kb/sec...'n")
    )
    .on('data', (d) ->
      file_stream.write d
      return
    )
    .on 'error', (e) ->
      callback(e)
      return
    .on 'end', () ->
      log("#{"[retis INFO]".green} Downloading #{url}...100%'n")
      logger.info("Downloaded #{url}.'n")
      # This is what causes the error
      callback()
      return
  return

在函数开始时运行回调有效，在函数结束时运行回调无效，但在end事件的回调中运行回调无效。

有人能建议如何修复我代码中的这个错误吗？

注意：您可以在https://github.com/jakhu/retis-ci在src/downloader.coffee文件中。请随意查看其他文件，以防此错误与它们有关。