mysql - error c# System.NullReferenceException -

i have code:

 public ordertotalmodel getordertotalmodelbyagentid(string agentids = null, datetime? fromdate = null, datetime? todate = null)     {         var session = sessionmanager.currentsession;         truncateamount truncateamount = new truncateamount();         string sql = @" select sum(o.totalquantity) totalquantity,sum(o.totalamount) totalamount                         orders o join workflows w on w.step = o.status                          inner join agents on a.id = o.agentid                                                     w.step = o.status                            , w.external = true , w.inactive = false                         , o.agentid in (:agentids) , o.approved = true";         //tu ngay         if (fromdate.hasvalue)         {             sql += " , date(o.created) >= date(:fromdate)";         }         // den ngay         if (todate.hasvalue)         {             sql += " , date(o.created) <= date(:todate)";         }         var sqlquery = session.createsqlquery(sql)               .addscalar("totalquantity", nhibernateutil.double)               .addscalar("totalamount", nhibernateutil.double);         sqlquery.setstring("agentids", agentids);         if (fromdate.hasvalue)         {             sqlquery.setdatetime("fromdate", (datetime)fromdate);         }         if (todate.hasvalue)         {             sqlquery.setdatetime("todate", (datetime)todate);         }         var result = sqlquery.list<object[]>().select(row => new ordertotalmodel()         {             totalquantity = (double)row[0],             totalamount = truncateamount.truncateamt((double)row[1]),         }).firstordefault();         return result;     } 

when input fromdate # todate return true when input fromdate == todate have error "system.nullreferenceexception". don't know problem, please me.

i think in case fromdate == todate sql function don't returns values. so, cannot call null.firstordefault()

 var result = sqlquery.list<object[]>().select(row => new ordertotalmodel()     {         totalquantity = (double)row[0],         totalamount = truncateamount.truncateamt((double)row[1]),     }).firstordefault();