diff --git a/sim_console.c b/sim_console.c
index 628a5afa..b665cb3a 100644
--- a/sim_console.c
+++ b/sim_console.c
@@ -2751,7 +2751,7 @@ return sim_ttcmd ();
 
 static t_bool sim_os_ttisatty (void)
 {
-return isatty (0);
+return isatty (fileno (stdin));
 }
 
 static t_stat sim_os_poll_kbd (void)