Strange bug in Wizard's Tower

[gimpz]

I got this bug in the third fight in the new wizard's tower challenge on Debir. The Caretaker has a unit of skeletons in that fight and every time they try to use their ability to create bones or whatever it's called, I get a popup message saying "You can't use that ability on such a powerful entity". I can click it away but it keeps coming up and only goes away if I spam click it many times. I managed to finish the fight but afterwards I get the popup again and then it doesn't matter how many times I click. I'm stuck with the popup message. Please help. I really want to complete that tower one day.'

EDIT: Problem solved by casting shackles on the skeletons and killing them early. Good to know if this happens to you ^^