iReport：使用XPath（XML数据源）的子报告显示为空白

I am using iReport 3.5.2 to create a report using an XML data source. I followed the "Step by step" example from the "Sub-reports" section of "The Ultimate Guide to iReport", and I was able to get the example working with SQL. However, when I try to use an XML data source and XPath, the sub-report comes out blank in the master report's rendition (though it renders fine by itself). I am using iReport 3.5.2 to create a report usi