PostgreSQL: NULL value in foreign key column

In my PostgreSQL database I have the following tables (simplified):

CREATE TABLE quotations (
  receipt_id bigint NOT NULL PRIMARY KEY
);

CREATE TABLE order_confirmations (
  receipt_id bigint NOT NULL PRIMARY KEY
  fk_quotation_receipt_id bigint REFERENCES quotations (receipt_id)
);

My problem now reads as follows:

I have orders which relate to previous quotations (which is fine 'cause I can attach such an order to the quotation referenced by using the FK field), but I also have placed-from-scratch orders without a matching quotation. The FK field would then be NULL, if the database let me, of course. Unfortunately, I get an error when trying to set fk_quotation_receipt_id to NULL in an INSERT statement because of a violated foreign key constraint.

When designing these tables I was still using PgSQL 8.2, which allowed NULL values. Now I've got 9.1.6, which does not allow for this.

What I wish is an optional (or nullable) foreign key constraint order_confirmations (fk_quotation_receipt_id) → quotations (receipt_id). I can't find any hints in the official PgSQL docs, and similar issues posted by other users are already quite old.

Thank you for any useful hints.

Works for me in 9.3 after correcting a missing comma. I'm sure it will work also in 9.1

create table quotations (
    receipt_id bigint not null primary key
);

create table order_confirmations (
    receipt_id bigint not null primary key,
    fk_quotation_receipt_id bigint references quotations (receipt_id)
);

insert into order_confirmations (receipt_id, fk_quotation_receipt_id) values 
    (1, null);

Confirmation will include:

INSERT 0 1