cursor empty

Hello,

I have got a function with this definition:

CREATE OR REPLACE FUNCTION vectorize(refcursor)
    RETURNS SETOF refcursor
    LANGUAGE 'plpgsql'
    COST 100
    STABLE
    ROWS 1000
AS $BODY$

begin
    perform pivottable(
        '_vector',
        'select * from viewdata',
        array['ideate', 'name', 'description', 'latitude', 'longitude'],
        array['parametername'],
        array['parametervalue::text', 'parametervaluetext']
    );
    open $1 scroll for select * from _vector;
    return next $1;
end

$BODY$;

The perform call creates a dynamic column pivot table, if I run manually

select pivottable(
    '_vector',
    'select * from viewdata',
    array['ideate', 'name', 'description', 'latitude', 'longitude'],
    array['parametername'],
    array['parametervalue::text', 'parametervaluetext']
);
select * from _vector;

I get all the data in the output, so everything is fine.

My goal is now to encapsulate the two lines into a function, so I define
a stable function and based on the dynamic column set a cursor. I get in
pgadmin the column names back, but the rows are empty if I run:

select * from vectorize('myvec');
fetch all from myvec;

Can you explain me, which part is wrong?
Thanks

Phil