Mistake of Fact

This was the intro to a chapter in my contracts book... from a Roman Text...

"If vinegar is sold as wine, bronze as gold, lead as silver, is there a sale?... If I think I am buying a virgin slave when she is not, the sale is valid. But if I sell a woman and you think you are buying a boy, since there is a mistake in sex, there is no sale."

Those guys were thinkers.