People v. Marrero N.Y. Court of Appeals 1987 Argued that his personal misunderstanding of a “peace officer” should get him off. Issue: Could the (D) misunderstanding of a definition allow him not to have the mens rea for this crime? Rule: Common law rule that says that ignorance of the law is no excuse. - The weapon possession statute can be violated irrespective of intent. Mistake of law defense is a viable exemption in those instances where individual demonstrates an effort to learn what law is, relies on validity of that law, and, later, it is determined that there was a mistake in the law itself. McKinney's Penal Law § 15.20 The statute that the (D) went on never was thought to be authorizing him to do what he did. He thought that he was an exemption.