'Can't set headers after they are sent' with express

Created by: jnjnljljjsw

my node.js version is 4.2.2

app.js: var api = require('./routes/api'); app.use('/api', api);

api.js proxy.on('proxyReq', function(proxyReq, req, res, options) { proxyReq.setHeader('X-Special-Proxy-Header', 'foobar'); }); router.all("/*", function(req, res){ proxy.web(req, res, { target: 'http://dev.lalocal.cn:8080/api' }); });

error: Error: Can't set headers after they are sent.