An exception of type 'System.IndexOutOfRangeException' occurred in System.Data.dll but was not handled in user code

 public IEnumerable<PraptraEntity> Get(string month, string year)
        {
            SqlConnection cn = new SqlConnection(ConfigurationManager.ConnectionStrings["conn"].ToString());
            SqlCommand cm = new SqlCommand();
            List<PraptraEntity> model = new List<PraptraEntity>();
            cm.Connection = cn;
            cm.Connection.Open();
            cm.CommandType = CommandType.StoredProcedure;
            cm.CommandText = "spGetpraptra104ByMonth";
            cm.Parameters.AddWithValue("@month", month);
            cm.Parameters.AddWithValue("@year", year);
            SqlDataReader rdr = cm.ExecuteReader();
            while (rdr.Read())
            {
                PraptraEntity praptraEntity = new PraptraEntity();
                //praptraEntity.pid = Convert.ToInt32(rdr["pid"]); praptraEntity.r_date = Convert.ToDateTime(rdr["r_date"]);// getting error
                praptraEntity.r_month = Convert.ToString(rdr["r_month"]);
                praptraEntity.DistrictName = Convert.ToString(rdr["DistrictHName"]);
                praptraEntity.cell_02 = Convert.ToString(rdr["cell_02"]);
                praptraEntity.cell_03 = Convert.ToDecimal(rdr["cell_03"]);
                praptraEntity.cell_04 = Convert.ToDecimal(rdr["cell_04"]);
                praptraEntity.cell_05 = Convert.ToDecimal(rdr["cell_05"]);
                praptraEntity.cell_06 = Convert.ToDecimal(rdr["cell_06"]);
                praptraEntity.cell_07 = Convert.ToDecimal(rdr["cell_07"]);
                praptraEntity.cell_08 = Convert.ToDecimal(rdr["cell_08"]);
                praptraEntity.cell_09 = Convert.ToDecimal(rdr["cell_09"]);
                praptraEntity.cell_10 = Convert.ToDecimal(rdr["cell_10"]);
                praptraEntity.cell_11 = Convert.ToString(rdr["cell_11"]);               
                model.Add(praptraEntity);
            }
            rdr.Close();
            cm.Connection.Close();
            return model;
        }            

Getting error  in this line kindly suggest: praptraEntity.r_date = Convert.ToDateTime(rdr["r_date"]);// getting error