java itext pdf 导出复杂表格，合并单元格问题

PdfPTable bTable = new PdfPTable(7);
PdfPCell cell = null;
cell = new PdfPCell(new PdfPCell(new Paragraph("日期",font2)));
cell = new Cell(new Cell(new Paragraph("日期",font2)));
cell.setRowspan(2);
cell = new PdfPCell(new PdfPCell(new Paragraph("类型",font2)));
cell.setRowspan(2);
cell.setColspan(2);//合并这个两行两列的时候老时报错，空指针异常