RIOT how to disable IRIs

[Laura Morales]

I have downloaded an RDF/XML file that looks like this


<?xml version="1.0" encoding="utf-8"?>
<rdf:RDF
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
    xmlns:owl="http://www.w3.org/2002/07/owl#";
    xmlns:foaf="http://xmlns.com/foaf/0.1/";
    xmlns:vann="http://purl.org/vocab/vann/";>
    <foaf:Document rdf:about="">
    <foaf:topic>
      <owl:Ontology>
        <vann:preferredNamespacePrefix>rdf</vann:preferredNamespacePrefix>
        
<vann:preferredNamespaceUri>http://www.w3.org/1999/02/22-rdf-syntax-ns#</vann:preferredNamespaceUri>
      </owl:Ontology>
    </foaf:topic>
  </foaf:Document>
</rdf:RDF>


the problem is that when I use RIOT to convert this to Turtle or N-Triples, all 
?subjects are converted to a <file://> IRI. Here's what I get in both formats


=====  NT


<file:///home/my/path/> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> 
<http://xmlns.com/foaf/0.1/Document> .
_:B5e821d41X2D5156X2D4835X2D87e5X2D6b024b963028 
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type> 
<http://www.w3.org/2002/07/owl#Ontology> .
<file:///home/my/path/> <http://xmlns.com/foaf/0.1/topic> 
_:B5e821d41X2D5156X2D4835X2D87e5X2D6b024b963028 .
_:B5e821d41X2D5156X2D4835X2D87e5X2D6b024b963028 
<http://purl.org/vocab/vann/preferredNamespacePrefix> "rdf" .
_:B5e821d41X2D5156X2D4835X2D87e5X2D6b024b963028 
<http://purl.org/vocab/vann/preferredNamespaceUri> 
"http://www.w3.org/1999/02/22-rdf-syntax-ns#"; .


=====  TTL


@prefix rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix owl:  <http://www.w3.org/2002/07/owl#> .
@prefix foaf:  <http://xmlns.com/foaf/0.1/> .
@prefix vann:  <http://purl.org/vocab/vann/> .

<file:///home/my/path/>
        rdf:type  foaf:Document .

_:b0    rdf:type  owl:Ontology .

<file:///home/my/path/>
        foaf:topic  _:b0 .

_:b0    vann:preferredNamespacePrefix  "rdf" ;
        vann:preferredNamespaceUri     
"http://www.w3.org/1999/02/22-rdf-syntax-ns#"; .

=====

For the life of me, I don't understand why these <file://> IRIs are being used, 
and how to remove them. I've tried adding --base=http://example.org to the RIOT 
arguments, but this seems to have no effect at all. Same output.
How can I disable <file://> IRIs in RIOT?