diff --git a/scrapy/core/downloader/handlers/http11.py b/scrapy/core/downloader/handlers/http11.py
index 4b02cb16f..b96c8c6fe 100644
--- a/scrapy/core/downloader/handlers/http11.py
+++ b/scrapy/core/downloader/handlers/http11.py
@@ -105,7 +105,7 @@ class TunnelingTCP4ClientEndpoint(TCP4ClientEndpoint):
         self._tunneledHost = host
         self._tunneledPort = port
         self._contextFactory = contextFactory
-        self._connectBuffer = b''
+        self._connectBuffer = bytearray()
 
     def requestTunnel(self, protocol):
         """Asks the proxy to open a tunnel."""