BUG #5199: Window frame clause wrong (?) behaviour

The following bug has been logged online:

Bug reference:      5199
Logged by:          Iliya Krapchatov
Email address:      my_working@inbox.ru
PostgreSQL version: 8.4.1
Operating system:   Windows XP
Description:        Window frame clause wrong (?) behaviour
Details:

I am new at WINDOW clause using so maybe I've missed something. Following is
the sequence of steps to repeat the problem:

create table t( pk integer );
insert into t(pk) values( 1 );
insert into t(pk) values( 2 );

select first_value( pk ) OVER ( pkw ) FROM t
WINDOW pkw AS ( PARTITION BY pk ORDER BY pk RANGE BETWEEN UNBOUNDED
PRECEDING AND UNBOUNDED FOLLOWING );

I see following lines in response:
---
ERROR:  cannot override frame clause of window "pkw"
LINE 1: select first_value( pk ) OVER ( pkw ) FROM t
                                      ^

********** Error **********

ERROR: cannot override frame clause of window "pkw"
SQL state: 42P20
Характеристика:31
---

I believe that is a bug because next two queries work fine:

select first_value( pk ) OVER ( PARTITION BY pk ORDER BY pk RANGE BETWEEN
UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING ) FROM t;

select first_value( pk ) OVER ( pkw ) FROM t
WINDOW pkw AS ( PARTITION BY pk ORDER BY pk );