This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Presumably, then, your context node wasn't the document element when you executed that element. You used a relative XPath expression there, which is relative to your context node. If you can't fix it to be relative to the actual context node at execution time, you could always use an absolute XPath expression.