OpenXml C#无法合并多个单元格

using DocumentFormat.OpenXml;
using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Spreadsheet;

public void MergeCells(string filePath, string sheetName, string startCell, string endCell)
{
    using (SpreadsheetDocument document = SpreadsheetDocument.Open(filePath, true))
    {
        WorkbookPart workbookPart = document.WorkbookPart;
        WorksheetPart worksheetPart = workbookPart.WorksheetParts.FirstOrDefault(wp => wp.PartName.Name == sheetName);

        if (worksheetPart != null)
        {
            Worksheet worksheet = worksheetPart.Worksheet;
            SheetData sheetData = worksheet.GetFirstChild<SheetData>();

            MergeCells mergeCells;
            if (worksheet.Elements<MergeCells>().Any())
            {
                mergeCells = worksheet.Elements<MergeCells>().First();
            }
            else
            {
                mergeCells = new MergeCells();
                worksheet.InsertAfter(mergeCells, sheetData);
            }

            MergeCell mergeCell = new MergeCell()
            {
                Reference = new StringValue($"{startCell}:{endCell}")
            };

            mergeCells.Append(mergeCell);
            worksheet.Save();
        }
    }
}