iTextSharp异常:“无法计算单元格的尺寸”

尝试运行使用iTextSharp库创建Pdf文档的简单示例(略有修改)。获取异常"无法计算单元格的尺寸"。

线"细胞。Left = 10f;"在valueCell函数中，抛出异常" Cell的尺寸无法计算"。注释这一行，一切正常。这个例外的原因是什么?

        private static Cell valueCell(string content)
        {
            var cell = new Cell(content);
            cell.BackgroundColor = new iTextSharp.text.Color(SystemColor.AliceBlue);
            cell.Left = 10f;
            return cell;
        }
        private static void TestPdfExport_iTextSharp()
        {
            var filePath = string.Format("D:''Temp''GeneratedPdf_iTs_{0}.pdf", DateTime.Now.ToString("yy-MM-dd hh mm ss"));
            // step 1: creation of a document-object
            Document document = new Document();
            try
            {
                // step 2: we create a writer that listens to the document
                // and directs a PDF-stream to a file
                PdfWriter writer = PdfWriter.GetInstance(document,
                                   new FileStream(filePath, FileMode.Create));
                // step 3: we open the document
                document.Open();
                // step 4: we create a table and add it to the document
                Table aTable = new Table(2, 2);    // 2 rows, 2 columns
                aTable.DefaultCell.Left = 10f;
                aTable.DefaultCell.Bottom = 10f;
                aTable.AddCell(valueCell("Metric"));
                aTable.AddCell(valueCell("Current Value"));
                aTable.AddCell(valueCell("Leverage"));
                aTable.AddCell(valueCell("3.2"));
                document.Add(aTable);   
            }
            catch (DocumentException de)
            {
            }

            // step 5: we close the document
            document.Close();
        }