pg 9.1.1 - ERROR: could not find pathkey item to sort

Is this a bug, or due to the way I am using the RANK() function in the
following query? The error is triggered when I use (sum(y) / sum(x))
in the ORDER BY of the RANK() function.

Simple test case:

create table t (a int, b int, x numeric, y integer );
insert into t
select g,round(mod(g,4)),
round((random()*1000)::NUMERIC,2),round((random()*1000)::NUMERIC) from
generate_series(1, 100) as g;

select a,
b,
(sum(y) / sum(x)) as res,
rank() over (partition by a,b order by (sum(y) / sum(x)) desc) as rank
--rank() over (partition by a,b order by sum(y) desc) as rank
-- no error
from t
group by a,b;

select version();
version
--------------------------------------------------------------------------------------------------------------
PostgreSQL 9.1.1 on x86_64-unknown-linux-gnu, compiled by gcc (GCC)
4.4.5 20110214 (Red Hat 4.4.5-6), 64-bit

Any other details needed?

Cheers,

Bricklen