First, claiming ignorance isn't a strong defense. It wouldn't hold up in any court in America, so I have no reason to buy it as an argument.

However, if it did, to be fair, you would have to seriously consider if it is reasonable to assume that person was ignorant. For example, if you know your friend is a drug dealer and he asks you to "drop off a bag at another house", you would get the book thrown at you even if you didn't know they were drugs in the bag.