diff -up cpython/Lib/imaplib.py.orig cpython/Lib/imaplib.py
--- cpython/Lib/imaplib.py.orig	2013-07-24 10:58:29.690668562 +0200
+++ cpython/Lib/imaplib.py	2013-07-24 10:58:11.583605735 +0200
@@ -156,7 +156,7 @@ class IMAP4:
     class abort(error): pass        # Service errors - close and retry
     class readonly(abort): pass     # Mailbox status changed to READ-ONLY
 
-    def __init__(self, host = '', port = IMAP4_PORT):
+    def __init__(self, host = None, port = IMAP4_PORT):
         self.debug = Debug
         self.state = 'LOGOUT'
         self.literal = None             # A literal argument to a command
@@ -237,7 +237,7 @@ class IMAP4:
     def _create_socket(self):
         return socket.create_connection((self.host, self.port))
 
-    def open(self, host = '', port = IMAP4_PORT):
+    def open(self, host = None, port = IMAP4_PORT):
         """Setup connection to remote server on "host:port"
             (default: localhost:standard IMAP4 port).
         This connection will be used by the routines:
@@ -1188,7 +1188,7 @@ if HAVE_SSL:
         """
 
 
-        def __init__(self, host='', port=IMAP4_SSL_PORT, keyfile=None, certfile=None, ssl_context=None):
+        def __init__(self, host=None, port=IMAP4_SSL_PORT, keyfile=None, certfile=None, ssl_context=None):
             if ssl_context is not None and keyfile is not None:
                 raise ValueError("ssl_context and keyfile arguments are mutually "
                                  "exclusive")
@@ -1208,7 +1208,7 @@ if HAVE_SSL:
             else:
                 return ssl.wrap_socket(sock, self.keyfile, self.certfile)
 
-        def open(self, host='', port=IMAP4_SSL_PORT):
+        def open(self, host=None, port=IMAP4_SSL_PORT):
             """Setup connection to remote server on "host:port".
                 (default: localhost:standard IMAP4 SSL port).
             This connection will be used by the routines:
@@ -1444,7 +1444,7 @@ if __name__ == '__main__':
             stream_command = val
             if not args: args = (stream_command,)
 
-    if not args: args = ('',)
+    if not args: args = (None,)
 
     host = args[0]