IndexError: list index out of range

hi i am try validate attachment file with pyubl i am getting following error:

DEBUG:spyne.model._base:        QualifyingPropertiesType: setting sub_ns='http://uri.etsi.org/01903/v1.3.2#'
DEBUG:spyne.model.complex:Got bases for QualifyingPropertiesType from orig: (<class 'spyne.model.complex.ComplexModelBase'>,)
Traceback (most recent call last):
  File "./ubl.py", line 45, in <module>
    sig_elt = data_elt.xpath('ds:Signature', namespaces={'ds': NS.XMLDSIG})[0];
IndexError: list index out of range

my code is same parse_schema.py