It's also very possible that you're using the Floppy version, which if it's like the Floppy version of King's Quest VI has 3 diskettes. Although it's weird that they'd say "disc" and not "disk".
I've played King's Quest V since a bit after King's Quest VI came out and I've gotten all kinds of errors other then that one. You might wanna check Ken William's official site <http://sierragamers.com/> though. I'll try to check it out myself.
Edit: Didn't find anything on the Internet.
You should update to the CD version, though I've heard that the Floppy version has a hilarious bug or something. Never got to see it.