Skolemization of RDF models with blank nodes as leaf nodes fails

[Aklakan]

2021-10-11 10:54:50,001 [Thread-540] WARN  o.a.s.l.c.m.MainCliLsq: Enrichment of PREFIX  xsd:  <http://www.w3.org/2001/XMLSchema#>
PREFIX  adhoc: <http://linked.opendata.cz/ontology/adhoc/>
PREFIX  rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#>

CONSTRUCT 
  { 
    _:c0 adhoc:class ?Class .
    _:c0 adhoc:endpointUri "http://pharmgkb.bio2rdf.org/sparql" .
    _:c0 adhoc:numberOfInstances ?numberOfInstances .
  }
WHERE
  { { SELECT  ?Class (COUNT(?resource) AS ?numberOfInstances)
      WHERE
        { ?resource  rdf:type  ?Class }
      GROUP BY ?Class
    }
  }
 failed
java.lang.RuntimeException: Leaf nodes must not be blank nodes: _:Ba09a0b35X2D2286X2D4138X2D9101X2D961dbe84fe69
	at org.aksw.jena_sparql_api.conjure.algebra.common.ResourceTreeUtils.createGenericHash(ResourceTreeUtils.java:238)
	at org.aksw.jena_sparql_api.conjure.algebra.common.ResourceTreeUtils.createGenericHash(ResourceTreeUtils.java:225)