Tutorial, Keeping Game Score

Ok, i followed this tutorial and tryed to make my own one
but there was a problem. When i pressed the button that increases with 100, the dynamatic field looked like this “NaN”
When i tryed to put a zero into it, it did like this ; 0 > 0100 > 0100100 and so on.
I have done exactly as the other one in the tutorial that works, but mine doesnt :S

Can anyone help me ?!