Python lxml / XPathEvalError : Invalid expression when I parse xml file

I try to parse an XML file with lxml. It works with some files but the last one gave me an error:

XPathEvalError at /admin/xml_exportation/xml/add/
Invalid expression

Here is the xml code :

<?xml version="1.0" encoding="utf-8" ?>
<export source="Source" version="1.0" date="2014-02-21T17:46:57">
  <Agence ID="XX1" externRef="XX1" customCode="38495">
    <Biens>
      <Bien ID="XX1-176" ref="XX1-176">
        <DateMAJ>2013-06-14T12:12:07</DateMAJ>
      </Bien>
    <Biens>
  </Agence>
</export>

And then the python code :

tree = etree.parse(xml_file) 
root_path = '/export/Agence/Biens/Bien/'
for element in tree.xpath(root_path): 
   print(element)

I tried to change the path but it gave me the same error.

Thank you for your help.