I am using this code to use the XMLRPC API to add stock to my inventory:

quant_id = models.execute_kw(database, uid, apikey, 'stock.quant', 'search', [[            ('product_id', '=', product_id), ('location_id', '=', location_id)]])

if quant_id:
     models.execute_kw(database, uid, apikey, 'stock.quant',  'write', [quant_id, {'quantity': newstocklevel}])
else:
     models.execute_kw(database, uid, apikey, 'stock.quant', 'create', [{                'product_id': product_id,'location_id': location_id,'quantity': newstocklevel}])

But this results in this error:

xmlrpc.client.Fault: <Fault 2: 'Hoeveelheden kunnen niet worden aangemaakt voor verbruiksartikelen of diensten.'>

Translated: quantities cannot be created for consumables or services

All my products are of type "goods" though.