What if a Yisrael does the עבודה of a לוי? (Erchin 11)

The Gemara states that a Levi who does someone else's job (sings instead of guarding the doors or vice versa) is either חייב מיתה or at least עובר a לאו. Also, a כהן who does a Levi's עבודה is עובר a לאו. However, interestingly enough the Minchas Chinuch points out that if a Yisrael does the עבודה of a לוי he is not חייב at all. For some reason there is no איסור for a Yisrael to do the עבודה of a לוי.