This is a very simple mistake, but it is a big one to make.

In English, the answers to positive and negative questions are the same. They do not change if the question becomes negative.

So, in English, there shouldn’t be “Yes, I’m not”, or “No, I am”.*
(* Actually, sometimes people do use “No, I am”, but it is to stress another person’s belief is wrong, not to answer the question. For example:
“You’re not going, right?”
“No, I am.” (= No, you’re wrong. I am going.))

Correct examples

“Are you stupid?”
“No (I’m not).”

“You’re stupid, aren’t you?”
“No (I’m not).”

“You’re not stupid, are you?
“No (I’m not).”