The addition functoid requires two or more numerical values as input and returns their sum as output. Also, strings respresenting numeric values will be processed, according to the documentation.
Please note that this functoid is not as forgiving as, for example, the string concatenation.
I have noticed that the addition functoid does not produce any output at all when one of the designated input tags:
- is not present in the source message, or
- has a non-numerical value
So, instead of producing 0 as result or ignoring the faulty input, no output is produced at all.