xpath "junk after end of expression" error

I'm trying to parse some XML output (a smart group) from the JSS API using xpath. The output is pretty standard stuff, a list of computers and any outstanding software updates on them. Let's save the entire file in a file ~/Desktop/whywontthiswork.xml.

In it are sections like this for each computer:

      <Available_SWUs>
        <Available_SWU>
          <Available_Update>Update 1</Available_Update>
        </Available_SWU>
        <Available_SWU>
          <Available_Update>Update 2</Available_Update>
        </Available_SWU>
        <Available_SWU>
          <Available_Update>Update 3</Available_Update>
        </Available_SWU>
        <Available_SWU>
          <Available_Update>Update 4</Available_Update>
        </Available_SWU>
      </Available_SWUs>

If I go at it with xpath to get at all the <Available_Update> elements I might use:

xpath ~/Desktop/whywontthiswork.xml '/advanced_computer_search/computers/computer/Available_SWUs/Available_SWU/Available_Update'

or even:

xpath ~/Desktop/whywontthiswork.xml '//Available_Update'

if I'm feeling lazy. Both of these will give me:

<Available_Update>Update 1</Available_Update>-- NODE --
<Available_Update>Update 2</Available_Update>-- NODE --
<Available_Update>Update 3</Available_Update>-- NODE --
<Available_Update>Update 4</Available_Update>-- NODE --

That's nice, but not terribly pretty.

Now comes the annoying bit. I BELIEVE xpath should remove the tags for me if I append () (a bit of sed trickery to gets rid of the -- NODE -- bits).

What I've been seeing is a line similar to (see here):

xpath ~/Desktop/whywontthiswork.xml '/advanced_computer_search/computers/computer/Available_SWUs/Available_SWU/Available_Update()' 2>&1 | sed 's/-- NODE --//g'

which, by all accounts, SHOULD return:

Update 1
Update 2
Update 3
Update 4

but instead taunts me with:

Parse of expression /advanced_computer_search/computers/computer/Available_SWUs/Available_SWU/Available_Update() failed - junk after end of expression: ( at /System/Library/Perl/Extras/5.18/XML/XPath/Parser.pm line 127.

which is annoying.