scotter老师，我这边遇到这个问题

这两句代码报出来的错

yield handler.call(this, next);

wechat.reply.call(this);

ReferenceError: handler is not defined

      at Object.<anonymous> (/Users/apple/Desktop/workspace/wechat/wechat/g.js:40:15)

      at Generator.next (<anonymous>)

      at onFulfilled (/Users/apple/Desktop/workspace/wechat/node_modules/co/index.js:65:19)

      at tryCatcher (/Users/apple/Desktop/workspace/wechat/node_modules/bluebird/js/release/util.js:16:23)

      at Promise._settlePromiseFromHandler (/Users/apple/Desktop/workspace/wechat/node_modules/bluebird/js/release/promise.js:512:31)

      at Promise._settlePromise (/Users/apple/Desktop/workspace/wechat/node_modules/bluebird/js/release/promise.js:569:18)

      at Promise._settlePromiseCtx (/Users/apple/Desktop/workspace/wechat/node_modules/bluebird/js/release/promise.js:606:10)

      at Async._drainQueue (/Users/apple/Desktop/workspace/wechat/node_modules/bluebird/js/release/async.js:138:12)

      at Async._drainQueues (/Users/apple/Desktop/workspace/wechat/node_modules/bluebird/js/release/async.js:143:10)

      at Immediate.Async.drainQueues (/Users/apple/Desktop/workspace/wechat/node_modules/bluebird/js/release/async.js:17:14)

      at runCallback (timers.js:666:20)

      at tryOnImmediate (timers.js:639:5)

      at processImmediate [as _immediateCallback] (timers.js:611:5)