Utah Code § 76-2-304: Ignorance or mistake of fact or law.

(1) Unless otherwise provided, ignorance or mistake of fact which disproves the culpable mental state is a defense to any prosecution for that crime.

(2) Ignorance or mistake concerning the existence or meaning of a penal law is no defense to a crime unless:

(a) due to an actor's ignorance or mistake, the actor reasonably believed the actor's conduct did not constitute an offense; and

(b) an actor's ignorance or mistake resulted from the actor's reasonable reliance upon:

(i) an official statement of the law contained in a written order or grant of permission by an administrative agency charged by law with responsibility for interpreting the law in question; or

(ii) a written interpretation of the law contained in an opinion of a court of record or made by a public servant charged by law with responsibility for interpreting the law in question.

(3) Although an actor's ignorance or mistake of fact or law may constitute a defense to the offense charged, the actor may nevertheless be convicted of a lesser included offense of which the actor would be guilty if the fact or law were as the actor believed.