fix wrong assertIs context
diff --git a/Lib/test/test_ftplib.py b/Lib/test/test_ftplib.py
index 9dc06cd..c8d0d88 100644
--- a/Lib/test/test_ftplib.py
+++ b/Lib/test/test_ftplib.py
@@ -738,7 +738,7 @@
 
         self.client.prot_p()
         sock = self.client.transfercmd('list')
-        self.assertIs(self.client.sock.context, ctx)
+        self.assertIs(sock.context, ctx)
         self.assertIsInstance(sock, ssl.SSLSocket)
         sock.close()