interval conversion bug

Hi,

I'm afraid that operator ::interval not working propertly
example:

select 1
######################
select now() + (1||' day')::interval;
working propertly
######################

I have my function writen in plpgsql

function test(varchar) returns varchar;

select 2
######################
select my_name,
test('12')
from table1
######################

returns 'aaa', '12 MONTH'

working propertly

select 3
######################
select my_name
now() + (test('12'))::interval
from table1
######################

not working with error "cannot cast type character varying to iterval"

Facts
Database version 7.4.5
psql version 7.4.5
platform PIV
linux : Debian

Regards

Andrzej Wojtaszek