C# .NET framework using Interop.Word and getting error 'Value cannot be null. Parameter name: pUnk'

I'm using Interop.Word to create a .docx file add some tables, fill it with data and then exporting it to a .pdf.

I keep getting this error:

System.ArgumentNullException: 'Value cannot be null.
Parameter name: pUnk'

All the online discussions regarding this issue are talking about TFS and VS version which I'm not really sure it is related to my problem at all.

I have several try catch in the code and I even commented the error. Yet, I can't put my hand on the reason for the exception!!

        //declare word app
        word.Application wordapp = new word.Application();
        object missing = Missing.Value;//NA Values
        word.Document myworddoc = null;

        if (File.Exists((string)filename))//check if it exists
        {
            object readOnly = false;//word settings
            object isvisible = false;
            wordapp.Visible = false;

            //word doc
            myworddoc = wordapp.Documents.Open(ref filename, ref missing, ref readOnly, ref missing, ref missing, ref missing
                , ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing
                , ref missing, ref missing);

            //open the word
            myworddoc.Activate();

            //replace key words
            importdata(wordapp,myworddoc);

            //export the file
            try
            {
                myworddoc.ExportAsFixedFormat(saveas.ToString(), word.WdExportFormat.wdExportFormatPDF);
            }
            catch (Exception)
            {
                XtraMessageBox.Show("Please close the opened File!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }

            // close the doc
            // closeWord();
            try
            {
                myworddoc.Close();
                wordapp.Quit();
            }
            catch (Exception)
            {
                closeWord();
            }

            XtraMessageBox.Show("Export Complete!", "Success", MessageBoxButtons.OK, MessageBoxIcon.Information);
        }
        else
        {
            XtraMessageBox.Show("Export Failed!", "Failure", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }

I get the exception immediately after the export is done successfully, no matter what the following line is.