With custom css style in xml, I am receiving below error when I select more than one quotation and click print. If I print single quotation everything is fine. If I remove custom css style from xml everything is working again. Please help me.

Traceback (most recent call last):
  File "/opt/odoo/odoo/addons/report/controllers/main.py", line 121, in report_download
    response = self.report_routes(reportname, docids=docids, converter='pdf')
  File "/opt/odoo/odoo/openerp/http.py", line 388, in response_wrap
    response = f(*args, **kw)
  File "/opt/odoo/odoo/addons/report/controllers/main.py", line 65, in report_routes
    pdf = report_obj.get_pdf(cr, uid, docids, reportname, data=options_data, context=context)
  File "/opt/odoo/odoo/openerp/api.py", line 237, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo/addons/report/models/report.py", line 269, in get_pdf
    paperformat, specific_paperformat_args, save_in_attachment
  File "/opt/odoo/odoo/openerp/api.py", line 237, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo/addons/report/models/report.py", line 480, in _run_wkhtmltopdf
    entire_report_path = self._merge_pdf(pdfdocuments)
  File "/opt/odoo/odoo/addons/report/models/report.py", line 569, in _merge_pdf
    writer.write(merged_file)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 264, in write
    self._sweepIndirectReferences(externalReferenceMap, self._root)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 339, in _sweepIndirectReferences
    self._sweepIndirectReferences(externMap, realdata)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 315, in _sweepIndirectReferences
    value = self._sweepIndirectReferences(externMap, value)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 339, in _sweepIndirectReferences
    self._sweepIndirectReferences(externMap, realdata)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 315, in _sweepIndirectReferences
    value = self._sweepIndirectReferences(externMap, value)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 324, in _sweepIndirectReferences
    value = self._sweepIndirectReferences(externMap, data[i])
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 339, in _sweepIndirectReferences
    self._sweepIndirectReferences(externMap, realdata)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 315, in _sweepIndirectReferences
    value = self._sweepIndirectReferences(externMap, value)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 354, in _sweepIndirectReferences
    newobj = self._sweepIndirectReferences(externMap, newobj)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 315, in _sweepIndirectReferences
    value = self._sweepIndirectReferences(externMap, value)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 315, in _sweepIndirectReferences
    value = self._sweepIndirectReferences(externMap, value)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 345, in _sweepIndirectReferences
    newobj = data.pdf.getObject(data)
  File "/usr/lib/pymodules/python2.7/pyPdf/pdf.py", line 649, in getObject
    retval = readObject(self.stream, self)
  File "/usr/lib/pymodules/python2.7/pyPdf/generic.py", line 67, in readObject
    return DictionaryObject.readFromStream(stream, pdf)
  File "/usr/lib/pymodules/python2.7/pyPdf/generic.py", line 519, in readFromStream
    key = readObject(stream, pdf)
  File "/usr/lib/pymodules/python2.7/pyPdf/generic.py", line 87, in readObject
    return NumberObject.readFromStream(stream)
  File "/usr/lib/pymodules/python2.7/pyPdf/generic.py", line 227, in readFromStream
    return NumberObject(name)
ValueError: invalid literal for int() with base 10: ''