I got this odoo server error whenever try printing multiple invoices at once.

Traceback (most recent call last):
File "/opt/odoo/addons/report/controllers/main.py", line 121, in report_download
response = self.report_routes(reportname, docids=docids, converter='pdf')
File "/opt/odoo/openerp/http.py", line 403, in response_wrap
response = f(*args, **kw)
File "/opt/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/openerp/api.py", line 241, in wrapper
return old_api(self, *args, **kwargs)
File "/opt/odoo/addons/report/models/report.py", line 274, in get_pdf
paperformat, specific_paperformat_args, save_in_attachment
File "/opt/odoo/openerp/api.py", line 241, in wrapper
return old_api(self, *args, **kwargs)
File "/opt/odoo/addons/report_rtl/models/report.py", line 196, in _run_wkhtmltopdf
content = self._merge_pdf(pdfdocuments)
File "/opt/odoo/addons/report/models/report.py", line 563, in _merge_pdf
pdfreport = file(document, 'rb')
TypeError: coercing to Unicode: need string or buffer, instance found

I've install both pypdf & pypdf2 by pip command tool and still got same error

my environment is: Server : Ubuntu 14.04.3 (32bit) & wkhtmltopdf 0.12.1 (with patched qt)

Any suggestion to solve this ??

My Regards to All