NULL value vs. DEFAULT value.

version = postgresql-8.4.4-2PGDG.el5.src.rpm

I am seeing this problem and I cannot explain why it is happening.
Evidently I misapprehend something about the interaction of NOT NULL
and DEFAULT. If someone could tell me what the actual case is I
would appreciate it very much.

The table definition looks like this:
CREATE TABLE ca_customs_shipments (
id integer NOT NULL,
. . .
weight_mass_gross numeric(14,4) DEFAULT 0.0 NOT NULL,
weight_mass_gross_uom character varying(3)
DEFAULT ' '::character varying NOT NULL,
weight_mass_net numeric(14,4) DEFAULT 0 NOT NULL,
weight_mass_net_uom character varying(3)
DEFAULT ' '::character varying NOT NULL,
. . .
);

My question is: Why am I getting a NULL exception? Should I only
specify DEFAULT and drop the NOT NULL constraint?

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