diff -r c6e0c29913ec Lib/imaplib.py
--- a/Lib/imaplib.py	Thu Sep 03 15:35:33 2015 -0700
+++ b/Lib/imaplib.py	Fri Sep 04 15:29:28 2015 +1200
@@ -273,7 +273,9 @@
 
 
     def _create_socket(self):
-        return socket.create_connection((self.host, self.port))
+        # Workaround for issue 18540:
+        host = None if str(self.host) == '' else self.host
+        return socket.create_connection((host, self.port))
 
     def open(self, host = '', port = IMAP4_PORT):
         """Setup connection to remote server on "host:port"
diff -r c6e0c29913ec Lib/test/test_imaplib.py
--- a/Lib/test/test_imaplib.py	Thu Sep 03 15:35:33 2015 -0700
+++ b/Lib/test/test_imaplib.py	Fri Sep 04 15:29:28 2015 +1200
@@ -241,6 +241,17 @@
             client.shutdown()
 
     @reap_threads
+    def test_issue18540(self):
+        # Test that the underlaying socket uses it's defaults if no arguments
+        # are given (instead of getting invalid arguments, see issue 18540).
+        try:
+            # Should succeed or raise a ConnectionRefusedError:
+            client = self.imap_class()
+            client.shutdown()
+        except ConnectionRefusedError:
+            pass
+
+    @reap_threads
     def test_issue5949(self):
 
         class EOFHandler(socketserver.StreamRequestHandler):