Potomac Leasing Co. leased an automatic telephone system to Vitality Centers. Claudene Cato signed the lease as guarantor of payments. When the rental was not paid, Potomac Leasing brought suit against Vitality and Cato. They raised the defence that the rented equipment was to be used for an illegal purpose— namely, the random sales solicitation by means of an automatic telephone in violation of state statute; that this purpose was known to Potomac Leasing; and that Potomac Leasing could therefore not enforce the lease. Was this defence valid?