strange behavior of openmc-validate-xml script

Hi,
Has any one experienced the message above with openmc-validate-xml python script applied to basic example as provided with openmc release 0.8.0.
OpenMC has been installed under Ubuntu 16.04.

tarek@ubuntu:~/OpenMC-2017/openmc/scripts$ python openmc-validate-xml -i /home/tarek/OpenMC-2017/openmc/examples/xml/basic/ -r /home/tarek/OpenMC-2017/openmc/src/relaxng/
Validating settings.xml…[VALID]
Validating tallies.xml…[VALID]
Validating materials.xml…[VALID]
Validating geometry.xml…Traceback (most recent call last):
File “openmc-validate-xml”, line 89, in
relaxng = etree.RelaxNG(relaxng_doc)
File “src/lxml/relaxng.pxi”, line 75, in lxml.etree.RelaxNG.init (src/lxml/lxml.etree.c:203201)
lxml.etree.RelaxNGParseError: Element material has a content type error, line 49

tarek@ubuntu:~/OpenMC-2017/openmc/scripts$

Tarek
Regards