通过openxml插入合并单元格提示错误：序列不包含任何元素 ，不知哪出的问题

[ahking]

   using (MemoryStream stream = new MemoryStream())
            {
                using (SpreadsheetDocument workbook = SpreadsheetDocument.Create(stream, SpreadsheetDocumentType.Workbook, true))
                {
                    var workbookPart = workbook.AddWorkbookPart();
                    workbook.WorkbookPart.Workbook = new DocumentFormat.OpenXml.Spreadsheet.Workbook();
                    workbook.WorkbookPart.Workbook.Sheets = new DocumentFormat.OpenXml.Spreadsheet.Sheets();

                    var sheetPart = workbook.WorkbookPart.AddNewPart<WorksheetPart>();
                    var sheetData = new DocumentFormat.OpenXml.Spreadsheet.SheetData();
                    sheetPart.Worksheet = new DocumentFormat.OpenXml.Spreadsheet.Worksheet(sheetData);

                    DocumentFormat.OpenXml.Spreadsheet.Sheets sheets = workbook.WorkbookPart.Workbook.GetFirstChild<DocumentFormat.OpenXml.Spreadsheet.Sheets>();
                    string relationshipId = workbook.WorkbookPart.GetIdOfPart(sheetPart);

                    uint sheetId = 1;
                    if (sheets.Elements<DocumentFormat.OpenXml.Spreadsheet.Sheet>().Count() > 0)
                    {
                        sheetId =
                            sheets.Elements<DocumentFormat.OpenXml.Spreadsheet.Sheet>().Select(s => s.SheetId.Value).Max() + 1;
                    }

                    DocumentFormat.OpenXml.Spreadsheet.Sheet sheet = new DocumentFormat.OpenXml.Spreadsheet.Sheet() { Id = relationshipId, SheetId = sheetId, Name = "Sheet1" };
                    sheets.Append(sheet);
                     //插入各项数据
                       //略..................
                    //开始合并单元格
                    string[] Letters = new string[] { "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", " " };
                    MergeCells mergeCells = new MergeCells();

                    MergeCell mergeCell;
                    /*
                    for (int i = 0; i < 13; i++)
                    {
                        mergeCell = new MergeCell() { Reference = Letters[i] + "1" + ":" + Letters[i] + "2" };
                        //mergeCell.
                        mergeCells.Append(mergeCell);
                    }*/
                    mergeCell = new MergeCell() { Reference = "A1:A2" };
                    //mergeCell.
                    mergeCells.Append(mergeCell);
                    sheet.InsertAfter(mergeCells, sheet.Elements<SheetData>().First());  
                   //这行报错
                    workbookPart.Workbook.Save();

                }