Commented Unassigned: Strange FileNotFoundException [9365]

I've got a worksheet with half a million rows. After writing values to cells in the first 20 rows. I call Save() on the workbook. That produces the following exception:

Could not find file 'C:\Users\nitew_000\AppData\Local\IsolatedStorage\dxiachod.3ue\sshwvvzf.hbs\Url.kxeclqde42apwq4041xfhhekihulkeam\Publisher.qolcwimjscmc2r1axln3vci10dth4a4y\Files\1vcczr3o.pnw'.

I just updated to version .74

Any ideas what file its looking for?
Comments: Got the latest source and debugged through it and the Exception is occurring in XLWorkbook_Save.cs on line 93. private void CreatePackage(String filePath, SpreadsheetDocumentType spreadsheetDocumentType) { PathHelper.CreateDirectory(Path.GetDirectoryName(filePath)); var package = File.Exists(filePath) ? SpreadsheetDocument.Open(filePath, true) : SpreadsheetDocument.Create(filePath, spreadsheetDocumentType); using (package) { CreateParts(package); //package.Close(); } <-- line 93 }