select distinct null

Hello!

I've got problem with such a query:

INSERT INTO table1 VALUES SELECT DISTINCT null,table2.atr1 FROM table2
WHERE table2.atr1 > 0;

It works in PostgreSQL 6.4.2 but in 7.0.3 it returns:

ERROR: Unable to identify an ordering operator '<' for type 'unknown'
Use an explicit ordering operator or modify the query

I discovered that PostgreSQL 7.0.3 doesn't accept the querry:

select distinct null;

and reurns the same error.

I use such queries in some client applications and it would be hard
to rewrite them. Could anyone explain to me what has been changed in 7.0.3
and how can I solve the problem on the server side.

Thanks in advance.

Karol Wieloch