Hello, I have Odoo 9. I want to call this method (it is in class account.account_invoice.py)

@api.v7    
def assign_outstanding_credit(self, cr, uid, id, credit_aml_id, context=None):
        credit_aml = self.pool.get('account.move.line').browse(cr, uid, credit_aml_id, context=context)        
        inv = self.browse(cr, uid, id, context=context) .........

When I do this

self.env['account.invoice'].assign_outstanding_credit(self.env.cr, self.env.uid, invoice.id, credit_aml_id, self.env.context)

I get error: 'NoneType' object is not callable.

Why? Thanks!