合并后设置单元格ddd边框只显示一半

按照网上说的,加了代码
((HSSFSheet)sheet).SetEnclosedBorderOfRegion(region11, NPOI.SS.UserModel.BorderStyle.Thick, NPOI.HSSF.Util.HSSFColor.Black.Index); 执行报错
代码如下:
static IWorkbook wk = new XSSFWorkbook();
ICellStyle cellStyle = wk.CreateCellStyle();
CellRangeAddress region11 = new CellRangeAddress(1, 2, 9, 10);
sheet.AddMergedRegion(region11);
row1.CreateCell(9).SetCellValue("Alarm");
row1.GetCell(9).CellStyle = styleTopBorder;