index was out of range. must be nonnegative and less than the size of the collection

protected void GridView1_RowDeleting(object sender, GridViewDeleteEventArgs e)
    {
               int task_id = Convert.ToInt32(GridView1.DataKeys[e.RowIndex].Value);//this Line Gives Error
        using (DailyWorkReportDBEntities1 context = new DailyWorkReportDBEntities1())
        {
            Task_Mst obj = context.Task_Mst.First(x => x.Task_Id == task_id);
            context.Task_Mst.DeleteObject(obj);
            context.SaveChanges();
            FillTask();
            ClientScript.RegisterStartupScript(this.GetType(), "alertMessage", "alert('Data Deleted');", true);
        }
    }