This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I'm not really sure what's causing your error message, but I'd recommend you abandon use of the logic resent and bean:size tags and simply use logic:empty and logic:notEmpty. This will simplify your logic, as it checks for both presence of the collection and a size of greater than zero.