XPointer & string-range with libxml2-python

Hi

I want to evaluate XPointer expressions with libxml2-python. It works 
well when a XPointer selects one or several nodes. However I do not 
succeed in evaluating XPointer that uses the function string-range.

For example, here is my xml file:
<?xml version = "1.0"?>
<ADDRBOOK>
        <ENTRY ID="pa">
                <NAME>Pieter</NAME>
        </ENTRY>
        <ENTRY ID="en">
                <NAME>Emeka</NAME>
        </ENTRY>
</ADDRBOOK>

And here is my code:
import libxml2
doc=libxml2.parseFile("addrbook.xml")
root=doc.children
ctx=root.xpointerNewContext(doc,None)
ctx.xpointerEval('xpointer(string-range(ADDRBOOK/ENTRY[1]/NAME[1],"Pieter"))')

It does not work and it gives me the following error:
Unable to convert XPath object type 7
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/usr/lib/python2.3/site-packages/libxml2.py", line 6443, in 
xpointerEval
    if ret is None:raise treeError('xmlXPtrEval() failed')
libxml2.treeError: xmlXPtrEval() failed

Is there something wrong in my code or in my XPointer ? Does the 
evaluation of that XPointer work if I use libxml2 directly ?

Moreover several xpointer-functions of xmlNode 
(xpointerNewCollapsedRange ...) do not work.
I have tried with an old version (python2.3.4 and libxml2-2.6.13) and 
with the latest(python2.4.1 and libxml2-2.6.19) but there have not been 
any improvements.