Error with DISTINCT and AS keywords

Hi,

I need some help to let this sql statement run with Postgres 7.2.1

Doesn't work:
select DISTINCT tnr, titel, 'TEST' AS testcol from tTitel;
ERROR:  Unable to identify an ordering operator '<' for type 'unknown'        Use an explicit ordering operator or
modifythe query
 

If I remove the DISTINCT keyword it works:
select tnr, titel, 'TEST' AS testcol from tTitel;



Greetings,
Andreas