You shouldn't be able to attack if you have no coal to fire your cannons with. I had recently been attacked by Sparky, which I successfully defnded...but left me with no coal. Krang attacked...with no coal...and since neither of us could do any damage, it was 4 draws - 1 for each attack.
Thus taking away my oh so precious brilliant strategist title
So, while not a big bug, it's annoying - a shouting match across the border is basically what we had, but it makes no sense that I now have 4 draws on my record (as does krang I would assume)