diff --git a/drivers/char/moxa.c b/drivers/char/moxa.c
index 65b6ff2442c6df4edfd839b37752ded8340df9c9..6799588b009976ca5d0efd98a065fd60f21d902d 100644
--- a/drivers/char/moxa.c
+++ b/drivers/char/moxa.c
@@ -1189,6 +1189,11 @@ static int moxa_open(struct tty_struct *tty, struct file *filp)
 		return -ENODEV;
 	}
 
+	if (port % MAX_PORTS_PER_BOARD >= brd->numPorts) {
+		retval = -ENODEV;
+		goto out_unlock;
+	}
+
 	ch = &brd->ports[port % MAX_PORTS_PER_BOARD];
 	ch->port.count++;
 	tty->driver_data = ch;
@@ -1213,8 +1218,8 @@ static int moxa_open(struct tty_struct *tty, struct file *filp)
 				moxa_close_port(tty);
 	} else
 		ch->port.flags |= ASYNC_NORMAL_ACTIVE;
+out_unlock:
 	mutex_unlock(&moxa_openlock);
-
 	return retval;
 }