mergeCells Usage

$objSheet ->mergeCells($gradeIndex."2".$endGradeIndex."2"); 执行不成功啊，总是报 Fatal error: Uncaught exception 'PHPExcel_Exception' with message 'Merge must be set on a range of cells.' in C:\wamp\www\lily\PHPExcel\PHPExcel\Worksheet.php on line 1703 到底该怎么写啊