Cannot use http in navigation menu

If site.xml contains a reference to an external link via an http: link the generation of th site fails:

<broken-links>
  <link message="No pipeline matched request: http:/forrest.apache.org/docs/plug
ins/index.xml" uri="http:/forrest.apache.org/docs/plugins/index.html">
    <referrer uri="linkmap.html"/>
    <referrer uri="samples/openoffice-writer-howto.html"/>
    <referrer uri="samples/index.html"/>
    <referrer uri="changes.html"/>
    <referrer uri="index.html"/>
    <referrer uri="todo.html"/>
    <referrer uri="samples/openoffice-writer.html"/>
    <referrer uri="samples/openoffice-impress.html"/>
  </link>
</broken-links>