Im using jasperReports with dynamicReports and I want to build a crosstab report. so far I have figured out that this error happens when I add columns that are numeric to rowGroups or columnGroups. this is what I get and I don't know why and I don't know how to solve this.

The error is: 

```
The bucket expression values are not comparable and no comparator specified

```

My code is:

```
    CrosstabValues crosstabValues = report.getCrosstab().getCrosstabValues();
    Collection<CrosstabRowGroupBuilder> rowGroup = generateRowGroup(crosstabValues);
    Collection<CrosstabColumnGroupBuilder> columnGroup = generateColumnGroup(crosstabValues);
    Collection<CrosstabMeasureBuilder> measures = generateMeasures(crosstabValues);

    CrosstabBuilder crosstab = ctab.crosstab();

    for(CrosstabRowGroupBuilder row : rowGroup)
        crosstab.addRowGroup(row);
    for(CrosstabColumnGroupBuilder columnGroupBuilder : columnGroup)
        crosstab.addColumnGroup(columnGroupBuilder);
    for(CrosstabMeasureBuilder measure : measures)
        crosstab.addMeasure(measure);

    crosstab.headerCell(cmp.text(crosstabValues.getHeader())
    .setStyle(getCrosstabHeaderCellStyle(report.getTemplate().getReportTemplateValues())));

```