This the result of my attempt to fill empty “published at” dates with “creation” dates.

Virtuoso 37000 Error SP031: SPARQL compiler: Variable 'entity' is not allowed in a constant clause

SPARQL query:
define sql:big-data-const 0
#output-format:text/html
define sql:signal-void-variables 1

INSERT DATA {
  GRAPH <http://mysite.eu/collection/published> {
    ?entity <http://mysite.eu/collection/published-at> ?created
    WHERE {
      SELECT ?entity ?created WHERE {
        GRAPH <http://mysite.eu/collection/published> {
          {
            ?entity <http://purl.org/dc/terms/issued> ?created .
            OPTIONAL { ?s <http://mysite.eu/collection/published-at> ?published . }
            FILTER(!bound(?published))
          }
        }
      }
    }
  }
}

What is wrong here? What does the mentioned constant clause? How to fix this?