How to use simple vocabulary line with GeoSPARQL:sfWithin and polygon?

Question

RDF:

prefix ex: <http://www.example.org/> 
prefix geo: <http://www.opengis.net/ont/geosparql#> 

ex:b7 a sf:Line ;
geo:asWKT "<http://www.opengis.net/def/crs/EPSG/0/4326> LINESTRING(55.3 -160.5,55.3 -160.5)^^geo:wktLiteral .

SPARQL query:

prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns>
prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
prefix geo: <http://www.opengis.net/ont/geosparql#> 
prefix geof: <http://www.opengis.net/def/function/geosparql/>
prefix sf: <http://www.opengis.net/ont/sf#> 
prefix ex: <http://www.example.org/> 
prefix ogis: <http://www.opengis.net/def/uom/OGC/1.0/>

select * where {
  ?a a sf:Line .
  FILTER (geo:sfWithin(?a, "<http://www.opengis.net/def/crs/EPSG/0/4326> POLYGON((50 -170, 50 -140, 60 -140, 60 -170))"^^geo:wktLiteral))
}

In open-source edition of Virtuoso this results in the error:

Virtuoso 42000 Error RDFGE: RDF box with a geometry RDF type and a non-geometry content

Does the line also need to be declared as a geo:Geometry?

Edit: sf:Line is a subclass of geo:Geometry, so I assumed it could be used in the same fashion.