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 modify the query

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

Greetings,
Andreas