linking beteen different documents

Hello DocBook users,

I try to link between different documents using olink. If I include the 
olink-element I get "????". The documents are all in the same directory.

This is the example source-file (bibliografie.xml):

<?xml version="1.0" encoding="UTF-8"?>
<bibliography xmlns="http://docbook.org/ns/docbook" version="5.0" 
xml:id="bibliografie">
    <title>Quellen</title>
    <bibliomixed xml:id="bib.bilbo">
        <bibliomset>
            <abbrev>BILBO</abbrev>
            <author>
                <personname>
                    <surname>Beutlin</surname>
                    <firstname>Bilbo</firstname>
                </personname>
            </author>
        </bibliomset>
    </bibliomixed>
</bibliography>


I use the following para-element:

<para>Text here... [<olink targetdoc="bibliografie.xml" 
targetptr="bib.bilbo"/>] Text...</para>

The result is <para>Text here... [????] Text...



Any ideas from the experts?

Thanks in advance,
Andreas