Problem with unique index

I have a table which I need to have 2 unique indexes on:  Drivers licence and Firearms Licence, my problem is, not
everyonehas a licence so I need the ability for the column to not be entered.  
 
However I get an error that I cannot insert a duplicate key into the unique index.  
Below is the view, rule and insert.  
As you can see I am excluding the 2 licences from the view, rule and insert, although they exist in the table.  
This is because in the application I test to see if it has been entered and then run a different sql insert depending
onthe data entered.  
 
I really need these to be unique values, I was sure that a null value would be ok, help please!

CREATE VIEW addperson_view
AS SELECT 
p.person_id, p.firstname, p.lastname, p.dob, p.street, p.suburb, p.city, p.homephone,  p.workphone, p.mobile, p.type,
p.date_approved,p.approved_by, p.vehicle_type,  p.vehicle_rego, p.notes, fp.location1, fp.location2, fp.location3
 
FROM person p, forest_person fp;

CREATE RULE addperson_rule AS ON INSERT TO addperson_view
DO INSTEAD(
INSERT INTO person
VALUES
(new.person_id, new.firstname, new.lastname, new.dob, new.street, new.suburb, new.city,  new.homephone, new.workphone,
new.mobile,new.type, new.date_approved, new.approved_by,  new.vehicle_type, new.vehicle_rego, new.notes);
 
INSERT INTO forest_person
VALUES
(new.person_id,new.location1,new.location2,new.location3);
);


user=>insert into
addperson_view
(person_id, firstname, lastname, dob, street, suburb, city, homephone, workphone, mobile, type, date_approved,
approved_by,vehicle_type, vehicle_rego, notes, location1, location2, location3)
 
values(nextval('seq_person_id'), 'Sharon', 'Cowling', '16-10-78', 'My Street', 'My Suburb', 'My City',
'5555568','5555567','0255556674','Other','09-01-02', 'test', 'Nissan','AAA121','Test Data
Notes','Forest1','Forest2','Forest3');
ERROR:  Cannot insert a duplicate key into unique index person_drivers_licence_key
(It also errors on firearms licence)

Table Structure:
CREATE TABLE person( 
person_id INT NOT NULL,
firstname VARCHAR(25) NOT NULL,
lastname VARCHAR(25) NOT NULL,
dob date NOT NULL,
street VARCHAR(50) NOT NULL,
suburb VARCHAR(50),
city VARCHAR(50) NOT NULL,
homephone INT,
workphone INT,
mobile INT,
type VARCHAR(30) NOT NULL,
date_approved DATE NOT NULL,
approved_by VARCHAR(50) NOT NULL,
vehicle_type VARCHAR(50),
vehicle_rego VARCHAR(6),
drivers_licence VARCHAR(10) UNIQUE,
firearms_licence VARCHAR(20) UNIQUE, 
notes VARCHAR(80),
PRIMARY KEY (person_id));



Regards,

Sharon Cowling