diff --git a/HISTORY.md b/HISTORY.md
index 5e36579..eea378e 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -1,6 +1,7 @@
 unreleased
 ==================
 
+  * Prevent writing a response after headers are sent while flushing the request
   * deps: 
     * debug@^4.4.3
     * statuses@^2.0.2
diff --git a/index.js b/index.js
index bf15e48..1c060fb 100644
--- a/index.js
+++ b/index.js
@@ -243,6 +243,11 @@ function getResponseStatusCode (res) {
 
 function send (req, res, status, headers, message) {
   function write () {
+    if (res.headersSent) {
+      debug('cannot %d after headers sent', status)
+      return
+    }
+
     // response body
     var body = createHtmlDocument(message)
 
diff --git a/test/test.js b/test/test.js
index 516e088..966b3e5 100644
--- a/test/test.js
+++ b/test/test.js
@@ -525,6 +525,17 @@ var topDescribe = function (type, createServer) {
   })
 
   describe('request started', function () {
+    it('should not respond after headers are sent while flushing the request', function (done) {
+      var server = createServer(function (req, res) {
+        finalhandler(req, res)()
+        res.end('respond not by finalhandler')
+      })
+
+      wrapper(request(server)
+        .post('/foo'))
+        .expect(200, 'respond not by finalhandler', done)
+    })
+
     it('should not respond', function (done) {
       var server = createServer(function (req, res) {
         var done = finalhandler(req, res)