Validator error? Omitting rdf:RDF

Hi,

This is probably just me being dense, but I can't figure out why the  
validator rejects this:

<rdf:Description xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax- 
ns#">
   <rdf:type rdf:resource="http://example.org/Foo"/>
</rdf:Description>

I expect it to produce this triple:

    [] rdf:type <http://example.org/Foo> .

But the validator complains:

    Error: Your document does not contain any RDF statement.

According to [1], the top-level rdf:RDF can be omitted if there's  
only one element inside. So what's wrong about the document above?

Thanks and best regards,
Richard

[1] http://www.w3.org/TR/rdf-syntax-grammar/#section-Syntax-complete- 
document