mutation in Trigger

Hi Team,Am a sql developer and new to oracle, i was assinged a trigger foram having two tablesTable1 : TestCols : ID | DESCTable2: TEST_CUSTCols : MID | STATUSand i want a update in third table "S_ID" when an insert | Delete | Udpate happends on Table1.Table3 : SIDCols : M_ID | STATUSCREATE OR REPLACE TRIGGER TRG_TESTAFTER INSERT OR UPDATE OR DELETEOF ID,DESCON TESTREFERENCING OLD AS OLD NEW AS NEWFOR EACH ROWBEGIN UPDATE S_ID SET STATUS ='Y' WHERE SID in (SELECT SID FROM TEST_CUST WHERE M_ID=:old."ID"); END;--But am gettting error "A mutation table is defined as a table that is changing"need your help please...