I went to choose my army and there are none listed

Then I notice the "Select A Book" selection field, prefilled with Storm of Arrows, but with scroll bars. I try to scroll and it won't let me.
Clearly, the poster only wants a battle in Storm of Arrows (or the game will only let armies from the same book fight).
I now can't fight this battle and I have no way to cancel it

I've PM'd Livipajo to explain and apologise

May I make the following suggestions:
1- If only armies from certain books can fight them then don't display the challenge to those that don't have those books. I figure that you must know what books I have from numerous files on my PC.
2- Change the small print to "Accepter may only use an army from Storm of Arrows" (assuming you don't do point 1)
3- Provide a mechanism to undo the acceptance and put the challenge back for anyone to accept (assuming you don't do point 1)
I expect you may get a lot of these (or maybe I'm especially dumb.... quite possibly).
Is there anyway you can undo the challenge, or worst case - remove it, because it won't be played

Thanks.