diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index c82b455..005cf16 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -207,7 +207,7 @@
         """
         self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
         self.sock.connect((self.host, self.port))
-        self.file = self.sock.makefile('r')
+        self.file = self.sock.makefile('rb')
 
 
     def read(self, size):