What is the burden of proving a defendant's lack of knowledge that possession of drug paraphernalia is prohibited?

Defendant argues that in the absence of a posting requirement, a defendant's lack of knowledge that possession of drug paraphernalia is prohibited would be an affirmative defense, which the defendant would have the burden of proving. He concludes that the posting requirement was intended to shift this burden of proof; thus, instead of the defendant having the burden of proving lack of knowledge, the People have the burden of proving posting (i.e., constructive knowledge). The flaw in this argument is that, "[a]s the old saying goes, 'Ignorance of the law is no excuse.' " (People v. Mills (1992) 6 Cal.App.4th 1278, 1291, 8 Cal.Rptr.2d 310.) There was no burden to be shifted.