null pointer exception

page http://www.w3.org/RDF/Validator/rdfval returns a NullPointerException for the following. Parsers should not get null pointer exception.
<?xml version="1.0" encoding="utf-8"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
      xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
      xml:base="http://physpics.com/namespace/pixa#">
  <rdfs:Class rdf:ID="Color">
    <rdfs:subClassOf rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Resource"/>
  </rdfs:Class>
  <rdfs:simpleType rdfs:name='colordata'>
    <rdfs:restriction rdfs:base='hexBinary'>
      <rdfs:length rdfs:value='6' />     
    </rdfs:restriction>
  </rdfs:simpleType>
  <rdf:Property rdf:ID="color">
    <rdfs:comment>Color swatch value for a picture</rdfs:comment>
    <rdfs:domain rdf:resource=""/> 
        <rdfs:comment> ? </rdfs:comment>
    <rdfs:range rdf:datatype="colordata" /> 
  </rdf:Property> 
</rdf:RDF>
The parse succeeds if the restriction is reduced to 
      <rdfs:restriction rdfs:base='hexBinary'/>
In summary, a length restriction for a hexBinary produces an exception.