XPath Builder throws error on valid JSON data

XPath Builder throws error on valid JSON data

I'm importing a valid JSON feed (checked at jsonlint.com) into XPath Builder, and it loads the XML Node Tree fine.  But when I select a node on the tree to execute it gives me this error:

Invalid XML/JSON format javax.xml.xpath.XPathExpressionException


What might be going wrong?  How can we fix it?

Thanks.