diff -r 4b3899e734d5 Lib/http/server.py
--- a/Lib/http/server.py	Mon Aug 15 11:21:08 2016 +1000
+++ b/Lib/http/server.py	Mon Aug 15 12:28:48 2016 +0800
@@ -466,7 +466,7 @@
             })
             body = content.encode('UTF-8', 'replace')
             self.send_header("Content-Type", self.error_content_type)
-            self.send_header('Content-Length', int(len(body)))
+            self.send_header('Content-Length', str(len(body)))
         self.end_headers()
 
         if self.command != 'HEAD' and body:
diff -r 4b3899e734d5 Lib/test/test_httpservers.py
--- a/Lib/test/test_httpservers.py	Mon Aug 15 11:21:08 2016 +1000
+++ b/Lib/test/test_httpservers.py	Mon Aug 15 12:28:48 2016 +0800
@@ -190,7 +190,7 @@
         res = self.con.getresponse()
         self.assertEqual(res.status, HTTPStatus.NOT_IMPLEMENTED)
 
-    def test_head_keep_alive(self):
+    def test_header_keep_alive(self):
         self.con._http_vsn_str = 'HTTP/1.1'
         self.con.putrequest('GET', '/')
         self.con.putheader('Connection', 'keep-alive')
@@ -243,6 +243,20 @@
         data = res.read()
         self.assertEqual(int(res.getheader('Content-Length')), len(data))
 
+    def test_error_content_length_type(self):
+        # Issue #27414: content-length value should be of type str instead of int
+        def send_header(instance, keyword, value):
+            self.assertIsInstance(keyword, str)
+            self.assertIsInstance(value, str)
+            super(self.request_handler, instance).send_header(keyword, value)
+        try:
+            prev = self.request_handler.send_header
+            self.request_handler.send_header = send_header
+            self.test_error_content_length()
+        finally:
+            # restore the previous method no matter succeed or fail
+            self.request_handler.send_header = prev
+
     def test_send_error(self):
         allow_transfer_encoding_codes = (HTTPStatus.NOT_MODIFIED,
                                          HTTPStatus.RESET_CONTENT)