Hi Dario,
I convert from numbers to strings, it's ok. but another error i have when i try http://localhost:8080/tutorial_demo/default.html, it show pop-up and said :

"29: 16 : 46: 37,250 ER addfield addfield() method can only add fields that are not part of the subscription schema. Please use a value greater than 15."

What are my fault in default.html file? Can u help me.?