Error in modify statement in Data step. ORA-01403

Question

I am using the following transaction table to update the master table. What I have observed is that when the transaction table is small the update happens smoothly. When the size increases the code fails with an ORA-01403 error. It doesnt make any sense to me as it does not seem logical. Any help or direction will be great.

data temp(index=(comp=(card_number cell_package_sk)));
        set import_data;
        keep card_number cell_package_sk SMS_STATUS SMS_SENT_DT LastUpdate;
run;

data master (drop=new_status new_date1 new_date2);
        set temp(rename=(SMS_STATUS= new_status SMS_SENT_DT=new_date1 LastUpdate=new_date2) in=a) key=comp / unique;
        modify master;
        select (_iorc_);
                when (%sysrc(_sok)) do;
                        if a then do;
                                if upcase(new_status) = "DELRD" then SMS_STATUS = "DELIVERED";
                                ELSE if upcase(new_status) = "DND" then SMS_STATUS = "NDNC";
                                ELSE if upcase(new_status) = "NO-DLR" then SMS_STATUS = "UNKNOWN";
                                ELSE SMS_STATUS = "FAILED";
                                SMS_SENT_DT = new_date1;
                                SMS_DELIVERED_DT = new_date2;
                                replace;
                        end;
                        else do;
                                _error_ = 0;
                        end;
                end;
                otherwise do;
                        _ERROR_ = 0;
                end;
        end;
run;

ERROR: Update/delete failed. The observation may have been changed or deleted since the time it was read. ORACLE execute error: ORA-01403: no data found.

Note that Master is oracle table while temp is a SAS data set. The master table has around 45 million records and transaction has 500 thousand.