ZabbixAPI zapi.usergroup.update Error -32602: Invalid params

[ik_zelf]

Hi,

what I am trying to do is to make oncall rotation by replacing users in a usergroup. For this I get the groupid of the specific usergroup and the userid for the users that are to be added to the standby group.
This I tried using pyzabbix
Most likely the problem is my lack of python skills...

#!/usr/bin/python

from pyzabbix import ZabbixAPI
zapi = ZabbixAPI("http://localhost/zabbix")
zapi.login("admin", "admin")

standbygroups = zapi.usergroup.get( output= ['usergrpid','name'],
                                filter= { "name": ["standby" ] }
                              )

standbygroup = standbygroups[0]
sbygrpid= standbygroup['usrgrpid']

newsbyusers= zapi.user.get ( output= ['userid','name']
                          , filter= { "name": ["Arjan"]}
                          )
newsbyuser= newsbyusers[0]
# replace current standby group members with new:
res= zapi.usergroup.update (usergrpid= sbygrpid
                      ,userids= newsbyusers
                      )

What I get is:

Traceback (most recent call last):
  File "bin/zab_auth.py", line 31, in <module>
    ,userids= newsbyusers
  File "/Library/Python/2.7/site-packages/pyzabbix/__init__.py", line 156, in fn
    args or kwargs
  File "/Library/Python/2.7/site-packages/pyzabbix/__init__.py", line 133, in do_request
    raise ZabbixAPIException(msg, response_json['error']['code'])
pyzabbix.ZabbixAPIException: ("Error -32602: Invalid params., Incorrect parameters for user group. while sending {'params': {'usergrpid': u'17', 'userids': [{u'userid': u'10', u'name': u'Arjan'}]}, 'jsonrpc': '2.0', 'method': 'usergroup.update', 'auth': u'10d28651473917e670c82a78f4fc19d7', 'id': 4}", -32602)

I must be doing something simple wrong. Can you point me in the right direction?
BTW: using zabbix-2.4.4