As a general rule, a corrupt intent is an essential element of this offense.322 But notwithstanding this rule, it is held that an officer charged with extortion can not excuse his act through mistake or ignorance of the law.323 To commit the offense of extortion the officer must act under color of his office;324 for if his act is merely the act of a private person and not in his official capacity, there is no ground for a criminal charge.325 Nor is it an offense if the unlawful fee be demanded or taken from a person not liable therefor.326 An officer receiving a higher fee than the law allows him commits extortion;327 or if he takes fees when not entitled to any, he is likewise guilty of the offense.328 But if a person voluntarily pays an officer a fee to which he is not by law entitled, with a knowledge that he was not bound to pay it, there is no offense in the officer taking it.328