title: Error incrementing crosstab dataset
body: <p>I have a data source that is composed of object { groupName, rowName, colName, value }. I'm trying to put the fields row,col,val in a crosstab, separated by group [the group has headers]. A collection of the data objects is being stored as the main datasource.</p>
  
  <p>I'm gettting the error when I try to do this: </p>
  
  <pre><code>net.sf.jasperreports.engine.JRRuntimeException: Error incrementing crosstab dataset
      net.sf.jasperreports.engine.fill.JRFillCrosstab$JRFillCrosstabDataset.customIncrement(JRFillCrosstab.java:762)
      net.sf.jasperreports.engine.fill.JRFillElementDataset.increment(JRFillElementDataset.java:193)
      net.sf.jasperreports.engine.fill.JRCalculator.calculateVariables(JRCalculator.java:162)
      net.sf.jasperreports.engine.fill.JRVerticalFiller.fillDetail(JRVerticalFiller.java:747)
  </code></pre>
  
  <p>Does anyone have suggestions to why this is happening or how to fix it?</p>