Sparql query - skos:notation does not exist for some classifications

Is it expected behavior ? For instance, I had to pass code retrieval to OPTIONAL to make this query work:

PREFIX skos:<http://www.w3.org/2004/02/skos/core#>
PREFIX xkos:<http://rdf-vocabulary.ddialliance.org/xkos#>

  SELECT ?label ?code {
    <http://stamina-project.org/codes/nafr2/naf> skos:prefLabel ?label .              
    OPTIONAL {  <http://stamina-project.org/codes/nafr2/naf> skos:notation ?code . }
  }

(more precisely, it was to make the itemDetails query work, because it gets the code of the classification the item belongs to).