[7.0] export product error

When i try to export to prestashop a new product i got this error:

 openerp.addons.connector.queue.worker: Traceback (most recent call last):
  File "/home/openerp/lp/openerp-connector/connector/queue/worker.py", line 122, in run_job
    job.perform(session)
  File "/home/openerp/lp/openerp-connector/connector/queue/job.py", line 460, in perform
    self.result = self.func(session, *self.args, **self.kwargs)
  File "/home/openerp/lp/prestashoperpconnect/prestashoperpconnect/unit/export_synchronizer.py", line 205, in export_record
    return exporter.run(binding_id, fields=fields)
  File "/home/openerp/lp/prestashoperpconnect/prestashoperpconnect/unit/export_synchronizer.py", line 79, in run
    result = self._run(*args, **kwargs)
  File "/home/openerp/lp/prestashoperpconnect/prestashoperpconnect/unit/export_synchronizer.py", line 160, in _run
    self.prestashop_id = self._create(record)
  File "/home/openerp/lp/prestashoperpconnect/prestashoperpconnect/unit/export_synchronizer.py", line 124, in _create
    return self.backend_adapter.create(data)
  File "/home/openerp/lp/prestashoperpconnect/prestashoperpconnect/unit/backend_adapter.py", line 154, in create
    self._export_node_name: attributes
  File "/usr/local/lib/python2.7/dist-packages/prestapyt/prestapyt.py", line 267, in add
    return self.add_with_url(self._api_url + resource, content, files)
  File "/usr/local/lib/python2.7/dist-packages/prestapyt/prestapyt.py", line 503, in add_with_url
    xml_content = dict2xml.dict2xml({'prestashop': content})
  File "/usr/local/lib/python2.7/dist-packages/prestapyt/dict2xml.py", line 119, in dict2xml
    root, _ = _process_complex(doc, data.items())
  File "/usr/local/lib/python2.7/dist-packages/prestapyt/dict2xml.py", line 71, in _process_complex
    nodes = _process(doc, tag, value)
  File "/usr/local/lib/python2.7/dist-packages/prestapyt/dict2xml.py", line 49, in _process
    nodelist, attrs = _process_complex(doc, tag_value.items())
  File "/usr/local/lib/python2.7/dist-packages/prestapyt/dict2xml.py", line 71, in _process_complex
    nodes = _process(doc, tag, value)
  File "/usr/local/lib/python2.7/dist-packages/prestapyt/dict2xml.py", line 51, in _process
    node.appendChild(child)
  File "/usr/lib/python2.7/xml/dom/minidom.py", line 112, in appendChild
    if node.nodeType == self.DOCUMENT_FRAGMENT_NODE:
AttributeError: 'NoneType' object has no attribute 'nodeType'

and no export is done.

thank you
Omar