problem using with-option

A newbie question, I'm afraid. Can anybody enlighten me as to why this 
works:

<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" name="test">
  <p:directory-list path="/some/path"/>
</p:declare-step>

and this doesn't:

<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" name="test">
  <p:directory-list>
    <p:with-option name="path" select="'/some/path'"/>
  </p:directory-list>
</p:declare-step>

I'm using Calabash 0.9.10. The error returned is:

Error  : file:/home/kevin/xproc-test/import-tldinfo.xpl:3: option path 
unbound on p:directory-list step named #ANON.85 and no default readable 
port.
Error  : Pipeline failed: err:XS0032: option path unbound on 
p:directory-list step named #ANON.85 and no default readable port.  It 
is a static error if no binding is provided and the default readable 
port is undefined.


Kevin Flynn