Created Unassigned: Problem opening created spreadsheet in open office [9417]

I haved used this example to create a spreadsheet. The resulting spreadsheet will open fine in the Microsoft Execel Previewer but gets a general i/o error when I try to open it in OpenOffice Calc.

var workbook = new XLWorkbook();
var worksheet = workbook.Worksheets.Add("Sample Sheet");
worksheet.Cell("A1").Value = "Hello World!";
workbook.SaveAs("HelloWorld.xlsx");

Any ideas as to what I am doing wrong.