Strange error message in hand-on challenge

I'm trying to complete a hands-on challenge. I've created a validation rule, and tested it, and it works as expected, but when I submit, they say "Challenge not yet complete.

We tried to escalate a high priority case but couldn’t save the record. Check the logic in your validation rule’s formula."

But when I escalate the high priority case, it works... so I don't understand...

#Trailhead Challenges