diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index 970b6a537f7..4e8a4bb6fa4 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -1353,7 +1353,7 @@ class _Authenticator:
     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 80e27f39730..824865680fa 100644
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -419,6 +419,26 @@ class ThreadedNetworkedTests(unittest.TestCase):
             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):
diff --git a/Misc/ACKS b/Misc/ACKS
index eb47ffda533..fae6e081cbc 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -603,6 +603,7 @@ Gerrit Holl
 Shane Holloway
 Rune Holm
 Thomas Holmes
+Craig Holmquist
 Philip Homburg
 Naofumi Honda
 Jeffrey Honig
diff --git a/Misc/NEWS b/Misc/NEWS
index b9950c3a271..03a3c0fefd5 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -13,6 +13,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #23779: imaplib raises TypeError if authenticator tries to abort.
+  Patch from Craig Holmquist.
+
 - Issue #23319: Fix ctypes.BigEndianStructure, swap correctly bytes. Patch
   written by Matthieu Gautier.