How to only pass specific solution bindings from the default graph to the VALUES clause of the federated query?

Question

In the following SPARQL query, solutions of both the variables, ?citizenship_uri and ?place_of_birth are passed to the VALUES clause of the federated query.

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

SELECT ?place_of_birth ?citizenship        
WHERE {
 [] person:citizenship ?citizenship_uri ;
    person:placeOfBirth ?place_of_birth .
                          
  SERVICE <https://publications.europa.eu/webapi/rdf/sparql> {
      ?citizenship_uri skos:prefLabel ?citizenship .
       FILTER(LANG(?citizenship) = "en")
  }
}

The below is the federated query performed by the RDFLib where both the solution bindings are present in the VALUES. However, the place_of_birth is not required here, since it is not used in the federated query.

# Federated query

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

SELECT REDUCED * WHERE {
                    ?citizenship_uri skos:prefLabel ?citizenship .
                    FILTER(LANG(?citizenship) = "en")
                  }VALUES (?citizenship_uri ?place_of_birth) {(<http://publications.europa.eu/resource/authority/country/GBR> "London")}

How can I capture the ?place_of_birth without it being included in the VALUES clause of the federated query?

I get the following error from the SPARQL endpoint if the unused solution bindings are included in the federated query's VALUES clause.

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