error on docx file conversion using doc.saveas method

I am writing code to convert docx file into a text file in ASP.net form using VB.net server side script.
My code is working fine in development environment. Where as I get an error "Object reference not set to an instence of an object" When published.

The error is coming in the line doc.SaveAs(newfilename, newfileformat)

Here is my code snippet

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Sub wrdfile(ByVal flpath As String, ByVal flnames As String, ByVal conv As String)
       Dim filepath As String = flpath
        Dim newfilename As Object = conv & flnames.ToString & ".txt"
        Dim newfileformat As Object = Microsoft.Office.Interop.Word.WdSaveFormat.wdFormatDOSText
        Dim wordapp As Microsoft.Office.Interop.Word.Application = New Microsoft.Office.Interop.Word.Application()
        newfilename = conv.ToString & flnames.ToString.Replace(".", "") & ".txt"
        Dim doc As Microsoft.Office.Interop.Word.Document = wordapp.Documents.Open(filepath, False)
        doc.SaveAs(newfilename, newfileformat)

        doc.Close()
        wordapp = Nothing
End sub
************************************************

here are the error details

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:


Line 293:        Dim doc As Microsoft.Office.Interop.Word.Document = wordapp.Documents.Open(filepath, False)
Line 294:        doc.SaveAs(newfilename, newfileformat)
Line 295:
Line 296:        doc.Close()
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Any help in resolving this is greatly appreciated

Thank You