All variables unbound if SERVICE SILENT fails

INSERT DATA {
_:b0 <http://rdfs.org/ns/void#sparqlEndpoint> <http://nonexistent>.
_:b1 <http://rdfs.org/ns/void#sparqlEndpoint> <http://linkeddata.uriburner.com/sparql>.
}

select ?ds ?ep ?s where {
?ds <http://rdfs.org/ns/void#sparqlEndpoint> ?ep.
SERVICE SILENT ?ep {
select ?s where { ?s ?p ?o } limit 1
}
}

----------------------------------------------------------
| ds   | ep                                       | s    |
==========================================================
| _:b0 | <http://linkeddata.uriburner.com/sparql> | _:b1 |
|      |                                          |      |
----------------------------------------------------------