diff --git a/drivers/net/usb/hso.c b/drivers/net/usb/hso.c
index 7482d0d5e2784fb4e34fc0050c73c26419e55dde..67eb8390cf07bf014559d524cb768ee1e2b9f32d 100644
--- a/drivers/net/usb/hso.c
+++ b/drivers/net/usb/hso.c
@@ -1723,6 +1723,10 @@ static int hso_serial_tiocmset(struct tty_struct *tty, struct file *file,
 		D1("no tty structures");
 		return -EINVAL;
 	}
+
+	if ((serial->parent->port_spec & HSO_PORT_MASK) != HSO_PORT_MODEM)
+		return -EINVAL;
+
 	if_num = serial->parent->interface->altsetting->desc.bInterfaceNumber;
 
 	spin_lock_irqsave(&serial->serial_lock, flags);