diff --git a/Lib/multiprocessing/connection.py b/Lib/multiprocessing/connection.py
index 996887cb7139428..710aba9685efda5 100644
--- a/Lib/multiprocessing/connection.py
+++ b/Lib/multiprocessing/connection.py
@@ -963,7 +963,7 @@ def answer_challenge(connection, authkey: bytes):
                 f'Protocol error, expected challenge: {message=}')
     message = message[len(_CHALLENGE):]
     if len(message) < _MD5ONLY_MESSAGE_LENGTH:
-        raise AuthenticationError('challenge too short: {len(message)} bytes')
+        raise AuthenticationError(f'challenge too short: {len(message)} bytes')
     digest = _create_response(authkey, message)
     connection.send_bytes(digest)
     response = connection.recv_bytes(256)        # reject large message