invalid doc, but why?

Hi,

I have the following V5.0 docbook document and any validator tells me that the inner <optional> element is invalid. But where does that info come from? All I can see from the xsd file is that a <phrase> is legal inside an <optional> and an <optional> is
legal inside a <phrase>.  

 

Not questioning the validity, just trying to figure out where the rule is located.

thanks,

--Tim Arnold

 

<chapter xmlns="http://docbook.org/ns/docbook" 

         xml:id="test" version="5.0">

  <info><title>my title</title></info>

  <section>

    <info><title>my section</title></info>

    <para>

       <optional><phrase><optional>maybe</optional></phrase>

       </optional>

    </para>

  </section>

</chapter>