hamster-cli with export fails

Created by: mabgh

hamster export html 2018-12-01 2018-12-31
  File "/usr/bin/hamster", line 391, in <module>
    getattr(hamster_client, command)(*args)
  File "/usr/bin/hamster", line 218, in export
    writer = reports.simple(facts, start_time, end_time, export_format)
  File "/usr/lib/python3.6/site-packages/hamster/reports.py", line 60, in simple
    writer.write_report(facts)
  File "/usr/lib/python3.6/site-packages/hamster/reports.py", line 101, in write_report
    self._finish(facts)
  File "/usr/lib/python3.6/site-packages/hamster/reports.py", line 339, in _finish
    self.file.write(Template(self.main_template).safe_substitute(data))
  File "/usr/lib64/python3.6/codecs.py", line 377, in write
    self.stream.write(data)
TypeError: string argument expected, got 'bytes'

with lines 339 in reports.py as

    data = Template(self.main_template).safe_substitute(data)
    print(data) 

data is class str and a correct html is printed to stdout