diff --git a/Lib/http/server.py b/Lib/http/server.py
index ea0e295d283..ca2dd507392 100644
--- a/Lib/http/server.py
+++ b/Lib/http/server.py
@@ -474,7 +474,7 @@ class BaseHTTPRequestHandler(socketserver.StreamRequestHandler):
             })
             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 --git a/Misc/NEWS.d/next/Library/2018-06-17-10-48-03.bpo-33663.sUuGmq.rst b/Misc/NEWS.d/next/Library/2018-06-17-10-48-03.bpo-33663.sUuGmq.rst
new file mode 100644
index 00000000000..b2a8e3cc00c
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-06-17-10-48-03.bpo-33663.sUuGmq.rst
@@ -0,0 +1 @@
+Convert content length to string before putting to header.