Wonderful.
A little unsolicited advice -- if this form is not in production (that is, no one has started using it yet) you should consider re-naming your fields. Formulas, conditional formatting, rules -- they are all so much easier to deal with when your fields have names like "Quantity" and "Price" instead of field12345. And, if you put this form in production like this and need to make a change in 6 months, you'll make yourself batty trying to remember what field25 is.
Also, so you know for the future, that error message was basically saying "I think you are telling me about a field you'd like me to select, and I can't find it". Everything between the --> and the <-- was what it was trying to evaluate. That's how I knew (aside from looking at the text you'd pasted) what the error was, since we use "/" in our XPath to select a field...