vb.net - Hexadecimal value 0x00, is an invalid character OpenXML SDK -

i have following function takes datatable , builds excel spreadsheet data using openxml sdk, , returns byte array. 9 out of ten times, works great, occasionally, error out following message:

'.', hexadecimal value 0x00, invalid character. 

here function:

private function buildexcelsheet(byref tbl datatable) byte()         dim memstream new memorystream         memstream.position = 0         dim spreadsheetdocument spreadsheetdocument = spreadsheetdocument.create(memstream, spreadsheetdocumenttype.workbook)          ' add workbookpart document.         dim workbookpart workbookpart = spreadsheetdocument.addworkbookpart         try             workbookpart.workbook = new workbook         catch ex exception             debug.writeline(ex.message)         end try           ' add worksheetpart workbookpart.         dim worksheetpart worksheetpart = workbookpart.addnewpart(of worksheetpart)()         try             worksheetpart.worksheet = new worksheet(new sheetdata())         catch ex exception             debug.writeline(ex.message)         end try          ' add sheets workbook.         dim sheets sheets = spreadsheetdocument.workbookpart.workbook.appendchild(of sheets)(new sheets())          ' append new worksheet , associate workbook.         dim sheet sheet = new sheet         try             sheet.id = spreadsheetdocument.workbookpart.getidofpart(worksheetpart)             sheet.sheetid = 1             sheet.name = "data"              sheets.append(sheet)         catch ex exception             debug.writeline(ex.message)         end try          dim sheetdata = worksheetpart.worksheet.getfirstchild(of sheetdata)()         try             integer = 0 tbl.rows.count - 1                 dim row datarow = tbl.rows(i)                 dim erow new spreadsheet.row                 ' loop through cols                 j integer = 0 tbl.columns.count - 1                     dim ecell new spreadsheet.cell                     ' ecell.datatype = spreadsheet.cellvalues.string                     if string.isnullorempty(row(j).tostring) = false                         ecell.cellvalue = new spreadsheet.cellvalue(row(j).tostring)                         ecell.datatype = new enumvalue(of cellvalues)(cellvalues.string)                     else                         ecell.cellvalue = new spreadsheet.cellvalue(" ")                     end if                     erow.appendchild(ecell)                 next                 sheetdata.appendchild(erow)              next         catch ex exception             debug.writeline(ex.message)         end try          try             workbookpart.workbook.save()         catch ex exception             debug.writeline(ex.message)         end try          try             ' close document.             spreadsheetdocument.close()         catch ex exception             debug.writeline(ex.message)         end try         memstream.position = 0         dim bytes byte() = memstream.toarray()          memstream.close()          return bytes     end function 

when check debug output, error occurring @ following line:

spreadsheetdocument.close() 

i assume has data being passed function, when @ data, i'm not seeing null characters.

it simple i'm overlooking, appreciated.

thanks.

edit:

here debug output around error:

a first chance exception of type 'system.threading.threadabortexception' occurred in mscorlib.dll exception of type 'system.threading.threadabortexception' occurred in mscorlib.dll not handled in user code first chance exception of type 'system.argumentexception' occurred in system.xml.dll '.', hexadecimal value 0x00, invalid character. first chance exception of type 'system.threading.threadabortexception' occurred in mscorlib.dll 

and here stacktrace:

>   compliance.dll!compliance.excel.buildexcelsheet(system.data.datatable tbl) line 118 basic     compliance.dll!compliance.excel.page_load(object sender, system.eventargs e) line 30 + 0x13 bytes   basic 

just aren't missing anything, here caller:

protected sub page_load(byval sender object, byval e system.eventargs) handles me.load         dim bytes byte()         try             call getrecords()         catch ex exception             debug.writeline("problem loading records export excel: " & ex.message)         end try          try             bytes = buildexcelsheet(tbl)         catch ex exception             debug.writeline("problem building excel spreadsheet: " & ex.message)         end try           response.clear()         response.contenttype = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"         response.addheader("content-disposition", string.format("attachment; filename={0}_{1}.xlsx", focustype, rectype))         try             response.binarywrite(bytes)         catch ex exception             debug.writeline(ex.message)         end try          response.end()      end sub