Hi Gordon,
you're right... OMG! 
It happened to some students with the JCloze, but then I noticed the problem and there they got a 5% minimum... I thought it could be the same problem here but I tried (without making a mistake) and I got a 10%... But I've just realized (after reading your post and revising the activity!) that the student made a mistake... and then, it is exactly as you've said! With a mistake you get a 0% if you "check" the activity after answering each item... so sorry for having annoyed you about that
