diff -r 2a14385710dc Doc/library/http.server.rst
--- a/Doc/library/http.server.rst	Tue Nov 22 01:25:55 2016 -0500
+++ b/Doc/library/http.server.rst	Tue Nov 22 16:06:10 2016 +0800
@@ -221,8 +221,9 @@
       Adds the HTTP header to an internal buffer which will be written to the
       output stream when either :meth:`end_headers` or :meth:`flush_headers` is
       invoked. *keyword* should specify the header keyword, with *value*
-      specifying its value. Note that, after the send_header calls are done,
-      :meth:`end_headers` MUST BE called in order to complete the operation.
+      specifying its value. *keyword* and *value* should both be :class:`str`.
+      Note that, after the send_header calls are done, :meth:`end_headers`
+      MUST BE called in order to complete the operation.
 
       .. versionchanged:: 3.2
          Headers are stored in an internal buffer.
diff -r 2a14385710dc Lib/http/server.py
--- a/Lib/http/server.py	Tue Nov 22 01:25:55 2016 -0500
+++ b/Lib/http/server.py	Tue Nov 22 16:06:10 2016 +0800
@@ -469,7 +469,7 @@
             })
             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 2a14385710dc Lib/test/test_httpservers.py
--- a/Lib/test/test_httpservers.py	Tue Nov 22 01:25:55 2016 -0500
+++ b/Lib/test/test_httpservers.py	Tue Nov 22 16:06:10 2016 +0800
@@ -235,6 +235,7 @@
         self.assertEqual(res.read(), 'Ärger mit Unicode'.encode('utf-8'))
 
     def test_error_content_length(self):
+        from unittest import mock
         # Issue #16088: standard error responses should have a content-length
         self.con.request('NOTFOUND', '/')
         res = self.con.getresponse()
@@ -243,6 +244,22 @@
         data = res.read()
         self.assertEqual(int(res.getheader('Content-Length')), len(data))
 
+        # Issue #27414: Content-Length value should be of type str instead of int
+        with mock.patch.object(
+                self.request_handler, 'send_header') as mock_send_header:
+            self.con.request('NOTFOUND', '/', headers={'Connection': 'close'})
+            res = self.con.getresponse()
+            self.assertEqual(res.status, HTTPStatus.NOT_FOUND)
+
+            for args in mock_send_header.call_args_list:
+                # there must be a Content-Length or the test above will fail
+                if args[0][0] == 'Content-Length':
+                    value = args[0][1]
+            self.assertIsInstance(value, str)
+
+            data = res.read()
+            self.assertEqual(int(value), len(data))
+
     def test_send_error(self):
         allow_transfer_encoding_codes = (HTTPStatus.NOT_MODIFIED,
                                          HTTPStatus.RESET_CONTENT)