Casting varchar to interval.?

I was hoping to cast a varchar to an interval.
How is this done?  Nobody has provided an answer to
this questing when asked in the past:

    http://archives.postgresql.org/pgsql-general/2002-07/msg01482.php

I have a table 'config' that contains:

    CREATE TABLE "public"."config" (
      "config_key" VARCHAR(64) NOT NULL,
      "config_value" VARCHAR(256) NOT NULL,
      PRIMARY KEY("config_key")
    ) WITH OIDS;

So I can do this:

    # SELECT config_value
    # FROM config
    # WHERE config_key = 'commission_withhold_interval';
     config_value
    --------------
     1 month
    (1 row)

But I can't do this:

    # SELECT config_value::interval
    # FROM config
    # WHERE config_key = 'commission_withhold_interval';

    ERROR:  cannot cast type character varying to interval

Why doesn't that work?

Dante

----------
D. Dante Lorenso
dante@lorenso.com