PostgreSQL nullable Foreign Key

I'm creating a postgreSQL table that has a foreign key that references itself, so it's a structure which is similar to a tree:

CREATE TABLE Person(
    ID serial PRIMARY KEY,
    Description text,
    Name varchar(5),
    ParentID serial,
    FOREIGN KEY (ParentID) REFERENCES Person(ID)
    );

The problem is that ParentID is automatically set to NOT NULL and therefore there can be no root in this tree. How do I make it nullable?