Exception from HRESULT: 0x800A03EC

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnexport_Click(object sender, EventArgs e)
        {
            // creating Excel Application

            Microsoft.Office.Interop.Excel._Application app = new Microsoft.Office.Interop.Excel.Application();

            // creating new WorkBook within Excel application

            Microsoft.Office.Interop.Excel._Workbook workbook = app.Workbooks.Add(Type.Missing);

            // creating new Excelsheet in workbook

            Microsoft.Office.Interop.Excel._Worksheet worksheet = null;

            // see the excel sheet behind the program

            app.Visible = true;

            // get the reference of first sheet. By default its name is Sheet1.

            // store its reference to worksheet

            worksheet = workbook.Sheets["Sheet1"];

            worksheet = workbook.ActiveSheet;
            // changing the name of active sheet

            worksheet.Name = "Exported from gridview";

            // storing header part in Excel

            for (int i = 1; i < dataGridView1.Columns.Count + 1; i++)
            {

                worksheet.Cells[1, i] = dataGridView1.Columns[i - 1].HeaderText;

            }

            // storing Each row and column value to excel sheet

            for (int i = 0; i < dataGridView1.Rows.Count - 1; i++)
            {

                for (int j = 0; j < dataGridView1.Columns.Count; j++)
                {

                    worksheet.Cells[i + 2, j + 1] = dataGridView1.Rows[i].Cells[j].Value.ToString();

                }

            }

            // save the application

            workbook.SaveAs("c:\\output.xls", Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                Type.Missing,
                Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive,
                Type.Missing, Type.Missing, Type.Missing, Type.Missing);



            // Exit from the application

            app.Quit();
        }
    }
}