Exception (HRESULT: 0x800AC472) when using Excel.Worksheet.Select after calling Excel.Workbook.SaveAs

I'm opening an xlsx file and saving each sheet into a csv file.

Here is the relevant code for the saving:

int i=0;
foreach (Excel.Worksheet s in app.ActiveWorkbook.Sheets)
{
    s.Select(true); // Error here

    String outfile = outputpath + "("+i+")" + outputfilename + ".csv";
    wkb.SaveAs(outfile, Excel.XlFileFormat.xlCSVMSDOS);

    ++i;
}

There is no problem with the output file name or path, and the output file(s) do not exist. It saves the first two sheets then crashes. I tried with a different input file with 4 sheets and it worked perfectly, so it has to do with the input file.

The exception:

System.Runtime.InteropServices.COMException was unhandled
  HResult=-2146777998
  Message=Exception from HRESULT: 0x800AC472
  Source=ExcelXlsx2Csv
  ErrorCode=-2146777998
  StackTrace:
       at Microsoft.Office.Interop.Excel._Worksheet.Select(Object Replace)
       at ExcelXlsx2Csv.Program.Main(String[] args) in c:\Users\Edward\Documents\Visual Studio 2013\Projects\ExcelXlsx2Csv\ExcelXlsx2Csv\Program.cs:line 109
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

Any hints appreciated!