diff --git a/man4/tty_ioctl.4 b/man4/tty_ioctl.4
index 07d2f0ade..78633b030 100644
--- a/man4/tty_ioctl.4
+++ b/man4/tty_ioctl.4
@@ -241,6 +241,9 @@ or
 Make the given terminal the controlling terminal of the calling process.
 The calling process must be a session leader and not have a
 controlling terminal already.
+For this case,
+.I arg
+should be specified as zero.
 
 If this terminal is already the controlling terminal
 of a different session group then the ioctl fails with