JFS SPARQL Query -- Problem with SELECT having COUNT

Hi,
I have attempted to write a SPARQL query that uses the aggregate function COUNT .. GROUP BY. The query is very simple, like

SELECT ?o COUNT(?s)

WHERE {

       ?s ?p ?o.

} group BY ?o

However, the JFS SPARQL service failed to execute the query.

It seems that the service does not support this aggregate function.

Please let me know if there is a way to get this work or there are any packages or newer versions that support SPARQL extensions or ARQ queries.

Thanks,
Vu