invoice.send() fails rather mysteriously after invoice.save()

[ExTechOp]

I've been testing building Holvi invoices from csv data exported from a database. That worked fine, the data got saved to Holvi correctly with invoice.save() and could then be sent manually from the webUI.

Now, when I tried to also do invoice.send() immediately after I had done invoice.save(), things went egg-shaped with:

Traceback (most recent call last):
  File "./holvi_makeinvoice.py", line 80, in <module>
    invoice.send()
  File "/usr/lib/python2.7/site-packages/holviapi/invoicing.py", line 66, in send
    stat = self.api.connection.make_put(url, payload)
  File "/usr/lib/python2.7/site-packages/holviapi/connection.py", line 71, in make_put
    return self._make_ppp('put', url, payload)
  File "/usr/lib/python2.7/site-packages/holviapi/connection.py", line 92, in _make_ppp
    raise ApiError(e.__str__(), response=e.response)  # six.u messes this up
  File "/usr/lib/python2.7/site-packages/holviapi/errors/__init__.py", line 22, in __init__
    self.error_details = self.response.json()
  File "/usr/lib/python2.7/site-packages/requests/models.py", line 805, in json
    return complexjson.loads(self.text, **kwargs)
  File "/usr/lib64/python2.7/site-packages/simplejson/__init__.py", line 516, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python2.7/site-packages/simplejson/decoder.py", line 374, in decode
    obj, end = self.raw_decode(s)
  File "/usr/lib64/python2.7/site-packages/simplejson/decoder.py", line 404, in raw_decode
    return self.scan_once(s, idx=_w(s, idx).end())
simplejson.scanner.JSONDecodeError: Expecting value: line 5 column 1 (char 4)

The error message is rather opaque, I would appreciate some instructions on how to start debugging this.

[Eero af Heurlin]

probably holvi side error, anyway the dangerous tests which send invoice after creating it pass for me.