"It wouldn't be invalid" isn't the worst double negative in the world but it would be valid to say that it was unpleasant to read it when you could have used a less misdirecting choice of prose that wouldn't have had such a negative effect on my reading comprehension. That is to say that I could have enjoyed it less but I certainly didnt enjoy it as much as i could have if you hadn't used the double negative when a single positive wasn't any further from reach.