[tryton] Remove contact mechanism from party with Proteus

[TPH]

Hello,

could somebody tell me please why this is not working and tell me whats the 
correct way :

Party = Model.get('party.party')
party = Party.find()

for p in party:
    if '0815' in p.phone:
        for n, c in  enumerate(p.contact_mechanisms):
            if c.value == '0815':
                p.contact_mechanisms.pop(n)
        p.save()


I working with Tryton 2.4 an Proteus

Error message: Fault 1: UserError Cannot change the party of an contact 
mechanism

Thanks