CHECK constraint removing brackets

Hi,

I notice this had been raised as a bug (and subsequently over-ruled) so I'm asking how I can achieve the following
businessrule. 

I have an order table which has an invoice_id column that links to an invoice table (an order can only have 1 invoice,
butone invoice can have multiple orders.) 

An order can have either an unconfirmed state, or any other state after it's been confirmed.  If an order has the state
unconfirmed,the invoice_id column must be null, as an invoice won't have been created yet.  If an order has any other
stateexcept unconfirmed, the invoice_id must not be null. 

With the above in mind, I decided on the following check to enforce this:

(state = 'Unconfirmed'::client.order_state AND invoice_id = NULL) OR (state != 'Unconfirmed'::client.order_state AND
invoice_id!= NULL) 

However PostgreSQL (8.4.2) converts this to the following:

state = 'Unconfirmed'::client.order_state AND invoice_id = NULL::integer OR state <> 'Unconfirmed'::client.order_state
ANDinvoice_id <> NULL::integer 

This allows both an order state of "unconfirmed" and a non-null invoice_id, and an order state of "confirmed" and a
NULLinvoice_id. 

How can I achieve the above?

Thanks,
Andy