Where payment of an annual fee is not made by the normal due date, a warning shall be sent to the holder of the plant variety certificate indicating that he shall incur the forfeiture of his rights if this payment, together with that of the late fee, is not made before the expiry of the period provided for in the third paragraph of article R. 623-31. The absence of a warning or any error contained therein shall not constitute grounds for the restoration of the rights of the owner of the plant variety certificate.