diff --git a/mitogen/fakessh.py b/mitogen/fakessh.py
index 00a9cd03..66bad698 100644
--- a/mitogen/fakessh.py
+++ b/mitogen/fakessh.py
@@ -282,7 +282,7 @@ def _fakessh_main(mitogen_, dest_context_id):
         die('Missing hostname')
 
     for opt, optarg in opts:
-        if 0 and opt == '-s':
+        if opt == '-s':
             subsystem = True
         else:
             LOG.debug('Warning option %s %s is ignored.', opt, optarg)
@@ -291,6 +291,9 @@ def _fakessh_main(mitogen_, dest_context_id):
     LOG.debug('opts: %r', opts)
     LOG.debug('args: %r', args)
 
+    if subsystem:
+        die('-s <subsystem> is not yet supported')
+
     dest = mitogen.master.Context(mitogen_.router, dest_context_id)
     control_handle, stdin_handle = dest.call_with_deadline(None, True,
         _start_slave, mitogen.context_id, args)