diff -r 7ed567ad8b4c Lib/imaplib.py
--- a/Lib/imaplib.py	Tue Mar 31 22:03:59 2015 +0200
+++ b/Lib/imaplib.py	Tue Mar 31 17:41:40 2015 -0400
@@ -1314,7 +1314,7 @@
     def process(self, data):
         ret = self.mech(self.decode(data))
         if ret is None:
-            return '*'      # Abort conversation
+            return b'*'     # Abort conversation
         return self.encode(ret)
 
     def encode(self, inp):
diff -r 7ed567ad8b4c Lib/test/test_imaplib.py
--- a/Lib/test/test_imaplib.py	Tue Mar 31 22:03:59 2015 +0200
+++ b/Lib/test/test_imaplib.py	Tue Mar 31 17:41:40 2015 -0400
@@ -341,6 +341,25 @@
             ret, data = client.login_cram_md5("tim", b"tanstaaftanstaaf")
             self.assertEqual(ret, "OK")
 
+    
+    @reap_threads
+    def test_aborted_authentication(self):
+
+        class MyServer(SimpleIMAPHandler):
+
+            def cmd_AUTHENTICATE(self, tag, args):
+                self._send_textline('+')
+                self.response = yield
+
+                if self.response == b'*\r\n':
+                    self._send_tagged(tag, 'NO', '[AUTHENTICATIONFAILED] aborted')
+                else:
+                    self._send_tagged(tag, 'OK', 'MYAUTH successful')
+
+        with self.reaped_pair(MyServer) as (server, client):
+            with self.assertRaises(imaplib.IMAP4.error):
+                code, data = client.authenticate('MYAUTH', lambda x: None)
+
     def test_linetoolong(self):
         class TooLongHandler(SimpleIMAPHandler):
             def handle(self):