Cast varchar to interval in plpgsql

Hi,

I need to create a PL/pgSQL function to add months (something like this):

CREATE
Function ADD_MONTHS
( timestamp, -- start date
int4 ) -- number of months (>0 or <0)
RETURNS timestamp -- new date
AS '
DECLARE
v_arg1 ALIAS FOR $1;
v_arg2 ALIAS FOR $2;
v_line2 varchar(100);
v_qq timestamp;
BEGIN
v_line2 := ltrim(to_char(v_arg2,''999999''),'' '')||'' month'';

select v_arg1+v_line2::interval
into v_qq;

RETURN v_qq;
END; -- Function ADD_MONTHS
' LANGUAGE 'plpgsql';
----------------------------------------

But get error message when try to call it:

>select add_months(now(),3);

>Cannot cast type 'varchar' to 'interval'

Any ideas?

Thanks.
Sergei Pohilko