ORA-04091: error

Hi all,

im geting ORA-04091: error when using a trigger to subtract ncopies values of table (rental ; after i insert a NEW record in it )
from cno value of table(copies) to give me the no of copies of a book remaining.

can someone help me out with this?
my tables and code is as folllows:



CID CNO BOOK
----- ---------- --------------------
A1 5 BOOK1
B1 10 BOOK2

rental;
Name Null? Type
------------------------------- ---
RID NOT NULL VARCHAR2(5)
CID VARCHAR2(5)
NCOPIES NUMBER(2)


1 CREATE OR REPLACE TRIGGER NO_COPIES
2 AFTER INSERT ON RENTAL
3 FOR EACH ROW
4 DECLARE
5 K VARCHAR2(5);
6 K1 VARCHAR2(5);
7 BEGIN
8 SELECT CNO INTO K FROM COPIES WHERE CID =
9 (SELECT CID FROM RENTAL WHERE RID = :NEW.RID);
10 SELECT NCOPIES INTO K1 FROM RENTAL WHERE RID = :NEW.RID;
12 IF K1 < K THEN
13 UPDATE COPIES SET CNO = K-K1
14 WHERE CID =
15 (SELECT CID FROM RENTAL WHERE RID = :NEW.RID);
16 END IF;
17* END;
18 /

Trigger created.

SQL> INSERT INTO RENTAL VALUES ('R1','A1',1);
INSERT INTO RENTAL VALUES ('R1','A1',1)
*
ERROR at line 1:
ORA-04091: table.RENTAL is mutating, trigger/function may not see it
ORA-06512: at NO_COPIES", line 5


any help shall be much appreciated!!