Problem with extract(epoch from interval ...

Hi,
I have a little problem with extract epoch from interval. It seems that 
the query works if the interval is in a string but not from DB field.
Could someone provide support.
Thanks in advance. Best regards, Kaloyan Iliev

db=# begin;
BEGIN
db=# SELECT version();
version
-------------------------------------------------------------------------------------------------------------
PostgreSQL9.1.6 on amd64-portbld-freebsd9.1, compiled by cc (GCC) 
 
4.2.1 20070831 patched [FreeBSD], 64-bit
(1 row)

db=# SELECT EXTRACT(EPOCH FROM INTERVAL '3 months'); date_part
-----------   7776000
(1 row)

db=# CREATE TABLE a( b interval);
CREATE TABLE
db=# INSERT INTO  a  VALUES ('1 month');
INSERT 0 1
db=# INSERT INTO  a  VALUES ('3 month');
INSERT 0 1
db=#  INSERT INTO  a  VALUES ('6 month');
INSERT 0 1
dbr=#  SELECT extract(EPOCH FROM INTERVAL b) FROM a;
ERROR:  syntax error at or near "b"
LINE 1: SELECT extract(EPOCH FROM INTERVAL b) FROM a;