Error in trigger: table is mutating

CREATE OR REPLACE TRIGGER  "TRANSACTION" 
before
update of  totalunitsbought on SIPHOLDER referencing new AS NEW old AS OLD
for each row
begin
  declare
    mportfolioname varchar2(20);
    mportfoliono number;
    moperationmode varchar2(8);
    mtransactionworth number;
    munits number;

    mnav number;

  begin
    select portfoliono into mportfoliono from sipholder where accno=:new.accno;
    select portfolioname into mportfolioname from sip_price 
      where portfoliono=mportfoliono;
    munits:= :new.totalunitsbought - :old.totalunitsbought;

    select nav into mnav from sip_price where portfoliono=mportfoliono;

   if :new.totalunitsbought>:old.totalunitsbought then
      moperationmode:='B';
      mtransactionworth:=munits*mnav;
   else
      moperationmode:='S';
      mtransactionworth:=munits*mnav;
   end if;

   insert into transaction
     (accno, ransdate, modeofoperation, units, transactionworth, portfoliono, 
      portfolioname, oldunits, newunits) 
    values
      (:new.accno, sysdate, moperationmode, munits, mtransactionworth, 
       :new.portfoliono, mportfolioname, :old.totalunitsbought, 
       :new.totalunitsbought);

  end;
end;

ERROR:

ERROR at line 1:
ORA-04091: table ORA-04091: table NETBANKING.SIPHOLDER is mutating,
trigger/function may not see it
ORA-06512: at "NETBANKING.TRANSACTION", line 12
ORA-04088: error during execution of trigger 'NETBANKING.TRANSACTION'
ORA-06512: at "NETBANKING.BUYUNITS_FIFTEEN", line 23
. is mutating, trigger/function may not see it
ORA-06512: at "SYS.DBMS_ISCHED", line 150
ORA-06512: at "SYS.DBMS_SCHEDULER", line 441
ORA-06512: at line 1