[Exist-open] Error SENR0001: attribute "xml:id" has no parent element

[Martin H. <mh...@uv...>]

HI there,

I must be doing something stupid here, but I can't for the life of me 
figure it out. I have a large collection of documents that begin like this:

<TEI xmlns="http://www.tei-c.org/ns/1.0" xml:id="B58001.scx">

and I'm running this XQuery against them in the admin client (eXist 
1.1.1.4311):

   declare default element namespace "http://www.tei-c.org/ns/1.0";

   for $t in //TEI[text/descendant::date[starts-with(./@when, '1858')]]
     return $t/@xml:id

but I get this error:

Error SENR0001: attribute "xml:id" has no parent element

I can't see what's wrong with the query, and I can't really see how an 
xml:id attribute could not have a parent. Anyone see what I'm missing here?

All help appreciated,
Martin