Virtuoso 37000 Error SP031: SPARQL compiler: Variable name is used in the BINDINGS clause but not in the query result set

[namedgraph]

Hi. I have a valid SPARQL query

PREFIX  skos: <http://www.w3.org/2004/02/skos/core#>

SELECT ?concept
WHERE
  { GRAPH ?graph
      { ?concept  skos:inScheme  $concept_scheme }
  }
VALUES $concept_scheme { <https://example.org#scheme> }

However in Virtuoso OS version 07.20.3240 (a1fd819) it returns an error:

Virtuoso 37000 Error SP031: SPARQL compiler: Variable name 'concept_scheme' is used in the BINDINGS clause but not in the query result set

What is the reason for that? And is there a workaround?

[HughWilliams]

we are looking into this ...