diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index 4d9df55..eb05dcb 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -1306,7 +1306,7 @@
     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 --git a/Lib/test/test_imaplib.py b/Lib/test/test_imaplib.py
index b34e652..96b4f32 100644
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -325,6 +325,25 @@
             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):