diff --git a/HISTORY.md b/HISTORY.md
index 9c5f528..1c5e71c 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -1,6 +1,7 @@
 unreleased
 ==================
 
+  * fix: prevent writing a response after headers are sent while flushing the request
   * fix: use the status code as the error body when it has no name
   * deps: 
     * debug@^4.4.3
diff --git a/index.js b/index.js
index e1e4a6e..28aa0a9 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 c463b3b..6dcc15c 100644
--- a/test/test.js
+++ b/test/test.js
@@ -536,6 +536,17 @@ 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)