Create Table Unique problem... bug?

Hi, I've a problem creating a table. My Sql
instruction is:

CREATE TABLE questiontypes (
  id integer NOT NULL,
  flags integer,
  label text,
  UNIQUE (id,flags)
);

Is it ok?? I must have 'flags' like to NULL... but
'id' is NOT NULL...
Now if I insert a row:

INSERT INTO questiontypes (id,flags,label) values
(2,null,'a');

and re-insert it... I can!!!! Now there are two
identical row, and UNIQUE??
I think postgres must occur an error? Is it Ok?

(The same problem is with:
INSERT INTO questiontypes (id,label) values (2,'a');
Is it ok?)
Thanks you, Mauro

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