Virtuoso 7.1.0 does not return correct datatype for language-specified strings

[micheldumontier]

Using http://affymetrix.bio2rdf.org/sparql

SELECT ?datatype
WHERE {
 ?s a http://bio2rdf.org/affymetrix_vocabulary:Probeset .
 ?s http://purl.org/dc/terms/title ?o .  
  FILTER isLiteral(?o)
  BIND(DATATYPE(?o) AS ?datatype) .
}
limit 1

returns an empty value, but is expected to return rdf:langString as per http://www.w3.org/TR/sparql11-query/#func-datatype

LANG(?o) does return the language string.