In French, you have to say "it's me". So it's not an obvious matter of simple logic what form to use, it's just usage (whose usage?).

If there's something you need to say, think of the audience you are aiming at, and say it in a form which will be clear to them. Don't expect people to decode it using some set of rules you learned at school. If it's ambiguous to your audience, it's your fault and not theirs. You should have said it another way.

If on the other hand, you don't want to communicate with an audience, shut the f** up and everybody will be content.