Null 'key' argument - Charts

I'm trying the tutorial at http://community.jaspersoft.com/wiki/creating-charts-and-subreports-jasp... and am getting the same error. 

This is using JasperReports 6.2 andjasper studio 6.2.

The graph in that tutorial is also a bar chart.

Are there any tricks for debugging this stuff?  Java stack traces into code you don't have sources to (org.jfree in this case) and exception messages like "null 'key' argument" are pretty useless.

Here's the jrxml generated from the example:

<?xml version="1.0" encoding="UTF-8"?>
<!-- Created with Jaspersoft Studio version 6.2.0.final using JasperReports Library version 6.2.0  -->
<!-- 2016-01-28T16:16:22 -->
<jasperReport xmlns="http://jasperreports.sourceforge.net/jasperreports" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://jasperreports.sourceforge.net/jasperreports http://jasperreports.sourceforge.net/xsd/jasperreport.xsd" name="GraphReport" pageWidth="595" pageHeight="842" columnWidth="555" leftMargin="20" rightMargin="20" topMargin="20" bottomMargin="20" uuid="c029557d-c3bc-47e9-a5ff-f7544e7265cb">
    <property name="com.jaspersoft.studio.data.sql.tables" value=""/>
    <property name="com.jaspersoft.studio.data.defaultdataadapter" value="Sample DB"/>
    <queryString>
        <![CDATA[select count(*), SHIPCITY from orders group by SHIPCITY]]>
    </queryString>
    <field name="C1" class="java.lang.Long"/>
    <field name="SHIPCITY" class="java.lang.String"/>
    <background>
        <band splitType="Stretch"/>
    </background>
    <title>
        <band height="350" splitType="Stretch">
            <property name="com.jaspersoft.studio.unit.height" value="pixel"/>
            <barChart>
                <chart evaluationTime="Report">
                    <reportElement x="0" y="0" width="555" height="350" uuid="eb78ea14-af53-4212-90f4-0186ec9effcf"/>
                    <chartTitle/>
                    <chartSubtitle/>
                    <chartLegend/>
                </chart>
                <categoryDataset>
                    <categorySeries>
                        <seriesExpression><![CDATA[$F{SHIPCITY}]]></seriesExpression>
                        <valueExpression><![CDATA[$F{C1}]]></valueExpression>
                        <labelExpression><![CDATA[$F{SHIPCITY}]]></labelExpression>
                    </categorySeries>
                </categoryDataset>
                <barPlot>
                    <plot/>
                    <itemLabel/>
                    <categoryAxisLabelExpression><![CDATA[" "]]></categoryAxisLabelExpression>
                    <categoryAxisFormat>
                        <axisFormat/>
                    </categoryAxisFormat>
                    <valueAxisFormat>
                        <axisFormat/>
                    </valueAxisFormat>
                </barPlot>
            </barChart>
        </band>
    </title>
</jasperReport>