set-returning function in pg 7.4.6

Hi,

I need to write a function returning a set of integer.
I'm basing this example on GetRows from http://techdocs.postgresql.org/guides/SetReturningFunctions

Here's the function:

create or replace function GetRows() returns setof int as
'
declare
    r record;
begin
    for r in EXECUTE ''select customer_id from customers'' loop
        return next r.customer_id;
    end loop;
    return;
end
'
language 'plpgsql';


When I call this function as

 select GetRows();

I get this error:

ERROR:  set-valued function called in context that cannot accept a set
CONTEXT:  PL/pgSQL function "getrows" line 5 at return next



What am I doing wrong?

Raph