PL/PgSQL bug?

Hi all.

I have noticed a strange bug/feature in PL/PgSQL language. Whenever I pass 'null' as one of the parameters, every
argumentbecomes 'null'. 

For example:

CREATE FUNCTION div_mod( int4, text, int4, bool, int2 ) RETURNS int4 AS '
BEGIN
IF $1 ISNULL THEN
  RETURN 2;
END IF;
RETURN 0;
END;
' LANGUAGE 'plpgsql';

Now perform:
mercury# select div_mod( 1, 'Test', null, 't' 10 );
 div_mod
-----------
             2

Although the function is called with $1 = 1, it is 'null' in this case. Actually all $n parameters are 'null'.
PostgreSQLversion is: 

mercury=# select version();
                        version
--------------------------------------------------------
 PostgreSQL 7.0.2 on alpha-dec-osf4.0f, compiled by cc
(1 row)

Is this a bug or a feature?

Nix.