Appellant was convicted for failing to register as a sex offender in violation of Penal Code section 290. Appellant registered at his residence in San Francisco, but did not register as a second residence the home of a friend in San Mateo where he often stayed while doing construction work at the home. On appeal, he argued that the trial court erred in instructing the jury that he wilfully failed to register even if he did not believe that he had acquired a second residence and was required to register in San Mateo. The appellate court here agreed and reversed. A defendant must have actual knowledge of the duties imposed by section 290 in order to willfully violate the statute. An omission is neither purposeful nor willing if it is based upon ignorance of the requirements of the law. The instructional error was not harmless beyond a reasonable doubt. There was substantial evidence that appellant was misinformed about when the need to register additional residences arises, and that he did not understand the meaning of residence as used in section 290.