diff --git a/pymysql/connections.py b/pymysql/connections.py
index 00605dd9..32b37bbf 100644
--- a/pymysql/connections.py
+++ b/pymysql/connections.py
@@ -920,10 +920,7 @@ def _request_authentication(self):
             ):
                 auth_packet = self._process_auth(plugin_name, auth_packet)
             else:
-                # send legacy handshake
-                data = _auth.scramble_old_password(self.password, self.salt) + b"\0"
-                self.write_packet(data)
-                auth_packet = self._read_packet()
+                raise err.OperationalError("received unknown auth swich request")
         elif auth_packet.is_extra_auth_data():
             if DEBUG:
                 print("received extra data")