[c#] Putting GridView data in a DataTable

I'm trying to save a GridView to a DataTable.

I have code which should in theory do it but I keep getting this error:

Column 0 doesn't exist

Here is my code:

protected void Button1_Click(object sender, EventArgs e)
    {
        DataTable dt = new DataTable();
        for (int j = 0; j < GridView1.Rows.Count; j++)
        {
            DataRow dr;
            GridViewRow row = GridView1.Rows[j];
            dr = dt.NewRow();
            for (int i = 0; i < row.Cells.Count; i++)
            {
                dr[i] = row.Cells[i].Text;
            }

            dt.Rows.Add(dr);
        }
    }

Any help on how to fix this?

user this full solution to convert gridview to datatable

 public DataTable gridviewToDataTable(GridView gv)
        {

            DataTable dtCalculate = new DataTable("TableCalculator");

            // Create Column 1: Date
            DataColumn dateColumn = new DataColumn();
            dateColumn.DataType = Type.GetType("System.DateTime");
            dateColumn.ColumnName = "date";

            // Create Column 3: TotalSales
            DataColumn loanBalanceColumn = new DataColumn();
            loanBalanceColumn.DataType = Type.GetType("System.Double");
            loanBalanceColumn.ColumnName = "loanbalance";


            DataColumn offsetBalanceColumn = new DataColumn();
            offsetBalanceColumn.DataType = Type.GetType("System.Double");
            offsetBalanceColumn.ColumnName = "offsetbalance";


            DataColumn netloanColumn = new DataColumn();
            netloanColumn.DataType = Type.GetType("System.Double");
            netloanColumn.ColumnName = "netloan";


            DataColumn interestratecolumn = new DataColumn();
            interestratecolumn.DataType = Type.GetType("System.Double");
            interestratecolumn.ColumnName = "interestrate";

            DataColumn interestrateperdaycolumn = new DataColumn();
            interestrateperdaycolumn.DataType = Type.GetType("System.Double");
            interestrateperdaycolumn.ColumnName = "interestrateperday";

            // Add the columns to the ProductSalesData DataTable
            dtCalculate.Columns.Add(dateColumn);
            dtCalculate.Columns.Add(loanBalanceColumn);
            dtCalculate.Columns.Add(offsetBalanceColumn);
            dtCalculate.Columns.Add(netloanColumn);
            dtCalculate.Columns.Add(interestratecolumn);
            dtCalculate.Columns.Add(interestrateperdaycolumn);

            foreach (GridViewRow row in gv.Rows)
            {
                DataRow dr;
                dr = dtCalculate.NewRow();

                dr["date"] = DateTime.Parse(row.Cells[0].Text);
                dr["loanbalance"] = double.Parse(row.Cells[1].Text);
                dr["offsetbalance"] = double.Parse(row.Cells[2].Text);
                dr["netloan"] = double.Parse(row.Cells[3].Text);
                dr["interestrate"] = double.Parse(row.Cells[4].Text);
                dr["interestrateperday"] = double.Parse(row.Cells[5].Text);


                dtCalculate.Rows.Add(dr);
            }



            return dtCalculate;
        }

protected void btnExportExcel_Click(object sender, EventArgs e)
{
    DataTable _datatable = new DataTable();
    for (int i = 0; i < grdReport.Columns.Count; i++)
    {
        _datatable.Columns.Add(grdReport.Columns[i].ToString());
    }
    foreach (GridViewRow row in grdReport.Rows)
    {
        DataRow dr = _datatable.NewRow();
        for (int j = 0; j < grdReport.Columns.Count; j++)
        {
            if (!row.Cells[j].Text.Equals("&nbsp;"))
                dr[grdReport.Columns[j].ToString()] = row.Cells[j].Text;
        }

        _datatable.Rows.Add(dr);
    }
    ExportDataTableToExcel(_datatable);
}

Copying Grid to datatable

        if (GridView.Rows.Count != 0)
        {
            //Forloop for header
            for (int i = 0; i < GridView.HeaderRow.Cells.Count; i++)
            {
                dt.Columns.Add(GridView.HeaderRow.Cells[i].Text);
            }
            //foreach for datarow
            foreach (GridViewRow row in GridView.Rows)
            {
                DataRow dr = dt.NewRow();
                for (int j = 0; j < row.Cells.Count; j++)
                {
                    dr[GridView.HeaderRow.Cells[j].Text] = row.Cells[j].Text;
                }
                dt.Rows.Add(dr);
            }
            //Loop for footer
            if (GridView.FooterRow.Cells.Count != 0)
            {
                DataRow dr = dt.NewRow();
                for (int i = 0; i < GridView.FooterRow.Cells.Count; i++)
                {
                    //You have to re-do the work if you did anything in databound for footer.  
                }
                dt.Rows.Add(dr);
            }
            dt.TableName = "tb";
        }