list of all non system synonym that have no objects associated with it

[aj_usa]

If I do this
SELECT ds.owner, ds.synonym_name
FROM dba_synonyms ds,all_objects do
WHERE ds.owner(+) = do.owner AND
do.object_name IS NULL AND
ds.synonym_name = do.object_name and
do.owner IN ('SYSTEM', 'SYS');
-- no rows selected.