I know it sounds pedantic but this bit me in the ass once. You are talking about a retrospective, not a post-mortem. I asked for a post-mortem because of a mistake with a product I was responsible for. The CEO set a meeting where he was asking pointed questions about if I still had faith in the product and finally asked straight out if I thought it would fail. Reason being he comes from pharma where post-mortem is an autopsy. It's not performed unless you have a corpse to dissect. I explained that in software engineer we have post-mortems about why coffee ran low. He was not amused.