What is the defense of illegality in the context of a modification agreement?

1 Of course, if such commissions were in fact illegal, this would be a complete defense to plaintiff's claim. It is significant that defendant did not interpose the defense of illegality to defeat plaintiff's claim, nor do they contend on appeal that the payment of such a commission is illegal. Furthermore, it may be noted that if plaintiff were in fact entitled to such a commission, there would be no consideration for the purported modification agreement. Peachy v. Witter, 131 Cal. 316, 319, 63 P. 468.