W3C validator can't process RDF/XML

I'm trying to describe a very basic metro train station map with stops and times. This RDF to Turtle converter can parse my XML, but the W3C validator throws: Error: Your document does not contain any RDF statement.

I can't figure out why my document isn't valid because it isn't some very special use case? My blank nodes are described as Resource, according to the specification. Or do I have to use nodeID for multiple blank nodes?

<?xml version="1.0"?>
<rdf:RDF 
    xmlns:rdf="http://ift.tt/1lyvrJb"
    xmlns:ex="http://example.com/">

    <rdf:Description rdf:about="http://ift.tt/VEzgYl">

        <ex:verbundenMit rdf:parseType="Resource">
            <ex:Haltestelle rdf:resource="http://ift.tt/1lyvqEQ" />
            <ex:Linie rdf:resource="http://ift.tt/VEzgYn" />       
            <ex:Zeit>2</ex:Zeit>            
        </ex:verbundenMit>

        <ex:verbundenMit rdf:parseType="Resource">
            <ex:Haltestelle rdf:resource="http://ift.tt/1lyvqEQ" />
            <ex:Linie rdf:resource="http://ift.tt/1lyvrJd" />       
            <ex:Zeit>7</ex:Zeit>            
        </ex:verbundenMit>      

    </rdf:Description>

</rdf:RDF>