It’s nothing you’re doing.
That would work fine if I had had the wpDamage field set to be numeric instead of alphanumeric. As it is, it has to be accessed as field[wpDamage].text instead and you can’t directly do math with it.
Easiest way around would be to declare a variable, set that variable to the current integer(field[wpDamage].text) and then add to that variable. At the end you would set the damage field text equal to the variable. That make sense?
Or, I can change it to a numeric field.