A defense based upon any of the provisions of this subchapter is an affirmative defense, except that mental illness is not an affirmative defense, but an alternative plea or finding that may be accepted under appropriate evidence, when the affirmative defense of insanity is raised or the plea of guilty but mentally ill is made.
(ILCS Chapter 720, Act 5, § 6-4) (1981 Code, § 42.06, 6-4)