On OpenERP 7, I am using wkhtmltopdf for my report. Usually it works as I want, but sometimes on some Purchase Order there will show an error like this in terminal:

ERROR ACL_160420 openerp.service.web_services: Exception: ('Webkit error', "The command 'wkhtmltopdf' failed with error code = -11. Message: No diagnosis message was provided")

Traceback (most recent call last):

File "/home/innotek/OE/server/7.0/openerp/service/web_services.py", line 712, in go

(result, format) = obj.create(cr, uid, ids, datas, context)

File "/home/innotek/OE/addons/7.0/report_webkit/webkit_report.py", line 340, in create

result = self.create_source_pdf(cursor, uid, ids, data, report_xml, context)

File "/home/innotek/OE/server/7.0/openerp/report/report_sxw.py", line 511, in create_source_pdf

return self.create_single_pdf(cr, uid, ids, data, report_xml, context)

File "/home/innotek/OE/addons/7.0/report_webkit/webkit_report.py", line 315, in create_single_pdf

pdf = self.generate_pdf(bin, report_xml, head, foot, htmls)

File "/home/innotek/OE/addons/7.0/report_webkit/webkit_report.py", line 178, in generate_pdf

_("The command 'wkhtmltopdf' failed with error code = %s. Message: %s") % (status, error_message))

except_osv: ('Webkit error', "The command 'wkhtmltopdf' failed with error code = -11. Message: No diagnosis message was provided")

2016-04-20 06:04:52,706 3043 ERROR ACL_160420 openerp.netsvc: Webkit error

The command 'wkhtmltopdf' failed with error code = -11. Message: No diagnosis message was provided

Traceback (most recent call last):

File "/home/innotek/OE/server/7.0/openerp/netsvc.py", line 296, in dispatch_rpc

result = ExportService.getService(service_name).dispatch(method, params)

File "/home/innotek/OE/server/7.0/openerp/service/web_services.py", line 654, in dispatch

res = fn(db, uid, *params)

File "/home/innotek/OE/server/7.0/openerp/service/web_services.py", line 760, in exp_report_get

return self._check_report(report_id)

File "/home/innotek/OE/server/7.0/openerp/service/web_services.py", line 738, in _check_report

netsvc.abort_response(exc, exc.message, 'warning', exc.traceback)

File "/home/innotek/OE/server/7.0/openerp/netsvc.py", line 71, in abort_response

raise openerp.osv.osv.except_osv(description, details)

except_osv: (u'Webkit error', u"The command 'wkhtmltopdf' failed with error code = -11. Message: No diagnosis message was provided")

What is the wrong part? on .py or .html or .xml or the other part?