diff --git a/scrapy/core/downloader/handlers/http11.py b/scrapy/core/downloader/handlers/http11.py
index ad3285a32..a4d5a28c8 100644
--- a/scrapy/core/downloader/handlers/http11.py
+++ b/scrapy/core/downloader/handlers/http11.py
@@ -92,9 +92,11 @@ class TunnelingTCP4ClientEndpoint(TCP4ClientEndpoint):
 
     def requestTunnel(self, protocol):
         """Asks the proxy to open a tunnel."""
-        tunnelReq = to_bytes(
-            'CONNECT %s:%s HTTP/1.1\r\n' % (
-                self._tunneledHost, self._tunneledPort), encoding='ascii')
+        tunnelReq = (
+            b'CONNECT ' +
+            to_bytes(self._tunneledHost, encoding='ascii') + b':' +
+            to_bytes(str(self._tunneledPort)) +
+            b' HTTP/1.1\r\n')
         if self._proxyAuthHeader:
             tunnelReq += \
                 b'Proxy-Authorization: ' + self._proxyAuthHeader + b'\r\n'