[Bug] Certification for Non-existent Skill? (University)

If it's a bug, it's a harmless one. This seems to be how you get advanced placement for the Mixologist branch of the Culinary career. You earn the certificate by sitting in on a certain number of skill classes and it doesn't seem to matter which skill they're for. That might be by design, so that sims who have already maxed the Mixology skill still have a way to get the certificate.