diff --git a/drivers/usb/serial/kl5kusb105.c b/drivers/usb/serial/kl5kusb105.c
index ed8531a64768..e72a0b45a707 100644
--- a/drivers/usb/serial/kl5kusb105.c
+++ b/drivers/usb/serial/kl5kusb105.c
@@ -330,8 +330,8 @@ static int klsi_105_prepare_write_buffer(struct usb_serial_port *port,
 	unsigned char *buf = dest;
 	int count;
 
-	count = kfifo_out_locked(&port->write_fifo, buf + KLSI_HDR_LEN, size,
-								&port->lock);
+	count = kfifo_out_locked(&port->write_fifo, buf + KLSI_HDR_LEN,
+				 size - KLSI_HDR_LEN, &port->lock);
 	put_unaligned_le16(count, buf);
 
 	return count + KLSI_HDR_LEN;