Cannot read property 'robot' of undefined with hubot-timezone

[gasolin]

input webby time in london

and got debug log with error:

[Wed Apr 13 2016 13:58:51 GMT+0800 (CST)] DEBUG Executing listener callback for Message '${message}'
[Wed Apr 13 2016 13:58:51 GMT+0800 (CST)] ERROR TypeError: Cannot read property 'robot' of undefined
  at http (/home/gasolin/webby-confbot/node_modules/webbybot/dist/src/response.js:224:18)
  at getTimezoneInfo (/home/gasolin/webby-confbot/node_modules/hubot-timezone/src/timezone.coffee:37:3)
  at sendLocalTime (/home/gasolin/webby-confbot/node_modules/hubot-timezone/src/timezone.coffee:77:5)
  at convertTime (/home/gasolin/webby-confbot/node_modules/hubot-timezone/src/timezone.coffee:92:5)
  at TextListener.callback (/home/gasolin/webby-confbot/node_modules/hubot-timezone/src/timezone.coffee:116:5)
  at executeListener (/home/gasolin/webby-confbot/node_modules/webbybot/dist/src/listener.js:116:19)
  at allDone (/home/gasolin/webby-confbot/node_modules/webbybot/dist/src/middleware.js:85:16)
  at /home/gasolin/webby-confbot/node_modules/webbybot/node_modules/async/lib/async.js:380:13
  at /home/gasolin/webby-confbot/node_modules/webbybot/node_modules/async/lib/async.js:52:16
  at iterate (/home/gasolin/webby-confbot/node_modules/webbybot/node_modules/async/lib/async.js:260:24)
  at Object.async.forEachOfSeries.async.eachOfSeries (/home/gasolin/webby-confbot/node_modules/webbybot/node_modules/async/lib/async.js:281:9)
  at Object.async.inject.async.foldl.async.reduce (/home/gasolin/webby-confbot/node_modules/webbybot/node_modules/async/lib/async.js:374:15)
  at /home/gasolin/webby-confbot/node_modules/webbybot/dist/src/middleware.js:91:22
  at nextTickCallbackWith0Args (node.js:420:9)
  at process._tickCallback (node.js:349:13)