Can a witness not express an opinion on a defendant's guilt?

A witness may not express an opinion on a defendant's guilt. The reason for the rule is not because guilt is the ultimate issue of fact for the jury; opinion testimony often goes to the ultimate issue. Opinions on guilt or innocence are inadmissible because they are of no assistance to the trier of fact. The trier of fact is as competent as the witness to weigh the evidence and draw a conclusion on the issue of guilt. (People v. Coffman and Marlow (2004) 34 Cal.4th 1, 77.)