Dumb SPARQL query problem

Hi,

Sorry to bother the list, but I'm stumped by what should be a simple 
SPARQL query.  When applied to the dbpedia end-point [1], this search:

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX dbpedia-owl: <http://dbpedia.org/ontology/>
SELECT *
WHERE {
     ?pers a foaf:Person .
     ?pers foaf:surname "Malik" .
     OPTIONAL {?pers dbpedia-owl:birthDate ?dob }
     OPTIONAL {?pers dbpedia-owl:deathDate ?dod }
     OPTIONAL {?pers dbpedia-owl:placeOfBirth ?pob }
     OPTIONAL {?pers dbpedia-owl:placeOfDeath ?pod }
}
LIMIT 100

yields no results. Yet if you drop the '?pers foaf:surname "Malik" .' 
clause, you get a result set which includes a Malik with the desired 
surname property.  I'm clearly being dumb, but in what way? :-)

(I've tried adding ^^xsd:string to the literal, but no joy.)

Thanks,

Richard
[1] http://dbpedia.org/sparql
-- 
*Richard Light*