Odoo Server Error

Traceback (most recent call last):
  File "/opt/odoo/odoo/addons/web/controllers/main.py", line 1673, in report_download
    response = self.report_routes(reportname, docids=docids, converter=converter)
  File "/opt/odoo/odoo/odoo/http.py", line 517, in response_wrap
    response = f(*args, **kw)
  File "/opt/odoo/odoo/addons/web/controllers/main.py", line 1614, in report_routes
    pdf = report.with_context(context).render_qweb_pdf(docids, data=data)[0]
  File "/opt/odoo/odoo/odoo/addons/base/models/ir_actions_report.py", line 674, in render_qweb_pdf
    bodies, html_ids, header, footer, specific_paperformat_args = self.with_context(context)._prepare_html(html)
  File "/opt/odoo/odoo/odoo/addons/base/models/ir_actions_report.py", line 308, in _prepare_html
    body_parent = root.xpath('//main')[0]
IndexError: list index out of range