InfoPath Dev

hello.

i dont understand why the formula does not update the result.
i have a bottom that update two field, initially empty, and an expression box that multiplies the field for the field B.
but when i update the value inside field A and B the formula display NaN (or 1.#qnan).
apparently the expression box see empty field (A and B)

any suggestion wba

thank you.

So you get an error when A has a value and B doesn't? Or am I misunderstanding?

sorry, is just because of my bad English.

simplifying, i want to populate a new sequence of an xml node with a button that have a rule: multiplies field A and field B.
initially field A and field B are empty. with the rule set inside the button i read data from an secondary data source (another xml file) and i populate field a and field B. an expression box calculates the result.

I hope I explained myself better.

thank you.