Problem with PL/pgSQL

   Hi

   I try to create the next function:

create function ttt() RETURNS OPAQUE AS '
BEGIN
RAISE EXCEPTION "Hello"
END' language 'plpgsql';

The result is:

ERROR:  Unrecognized language specified in a CREATE FUNCTION: 'plpgsql'.  
Recognized languages are sql, C, internal and the created procedural languages.

   I have postgres SQL ver. 6.5.2. 

   What's going on?
Vladimir