'LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression

//var data = context.SpotBillOutputData
           //    .Join(context.SpotBillInputData,O=>O.CONS_REF,I=>I.CONS_REF,(O,I)=>new {O,I})
           //   .Join(context.SpotBillInputData, OB => OB.O.BILL_MTH, IB => IB.BILL_MTH.ToString(), (OB, IB) => new { OB, IB })
           //    .Where(c=>c.OB.O.BILL_MTH==MaxBillMonth && c.IB.CSTS_CD=="R" && c.OB.O.SDO_CD==SDO &&c.OB.O.BINDER==Binder)
           //    .Select(c => new Consumer { SDO_CD = c.OB.O.SDO_CD, BINDER = c.OB.O.BINDER, ACC_NO = c.OB.O.ACC_NO, NAME = c.IB.NAME, ADDR1 = c.IB.ADDR1, ADDR2 = c.IB.ADDR2, ADDR3 = c.IB.ADDR3, TRF_CD = c.IB.TRF_CD, CSTS_CD = c.OB.O.CSTS_CD, CONN_LOAD = c.IB.CONN_LOAD, METER_NO = c.IB.METER_NO, OPNRDG = c.IB.OPNRDG, OPNRDGSTS = c.IB.OPNRDGSTS }).ToList<Consumer>();


           var data = (from c in context.SpotBillOutputData
                       join
                           i in context.SpotBillInputData on new { CONS_REF = c.CONS_REF, BILL_MTH = c.BILL_MTH } equals new { CONS_REF = i.CONS_REF, BILL_MTH = Convert.ToString(i.BILL_MTH) }

                       where c.CSTS_CD == "R" && c.SDO_CD == SDO && c.BINDER == Binder && c.BILL_MTH == MaxBillMonth

                       select new Consumer { SDO_CD = c.SDO_CD, BINDER = c.BINDER, ACC_NO = c.ACC_NO, NAME = i.NAME, ADDR1 = i.ADDR1, ADDR2 = i.ADDR2, ADDR3 = i.ADDR3, TRF_CD = i.TRF_CD, CSTS_CD = c.CSTS_CD, CONN_LOAD = i.CONN_LOAD, METER_NO = i.METER_NO, OPNRDG = i.OPNRDG, OPNRDGSTS = i.OPNRDGSTS }).ToList<Consumer>();