unique constraints

PostgreSQL 9.5

I have a table:

CREATE TABLE test
(
id integer NOT NULL,
tag_id integer,
CONSTRAINT test_id_tag_id_key UNIQUE (id, tag_id)
)
WITH (
OIDS=FALSE
);

I noticed I can insert this into the database:

Insert into test (id, tag_id) values(1,null);
Insert into test (id, tag_id) values(1,null);

But this does not work:

Insert into test (id, tag_id) values(2,1);
Insert into test (id, tag_id) values(2,1); (error occurs)

Select * from test;

1, null
1, null
2, 1

I understand why in the second set of inserts I get an error when trying to insert the same set of values. But why does the first example work? I would have thought that it would have only allowed the first insert to work.

Thanks,

Lance