Error when an invoice is created referencing a PO line that has no product

[JordiBForgeFlow]

OpenERP Server Error
Client Traceback (most recent call last):
File "/home/opt/odoo/odoo70-prd/odoo/addons/web/http.py", line 204, in dispatch
response["result"] = method(self, *_self.params)
File "/home/opt/odoo/odoo70-prd/odoo/addons/web/controllers/main.py", line 1137, in call_button
action = self._call_kw(req, model, method, args, {})
File "/home/opt/odoo/odoo70-prd/odoo/addons/web/controllers/main.py", line 1125, in _call_kw
return getattr(req.session.model(model), method)(_args, **kwargs)
File "/home/opt/odoo/odoo70-prd/odoo/addons/web/session.py", line 42, in proxy
result = self.proxy.execute_kw(self.session._db, self.session._uid, self.session._password, self.model, method, args, kw)
File "/home/opt/odoo/odoo70-prd/odoo/addons/web/session.py", line 30, in proxy_method
result = self.session.send(self.service_name, method, *args)
File "/home/opt/odoo/odoo70-prd/odoo/addons/web/session.py", line 103, in send
raise xmlrpclib.Fault(openerp.tools.ustr(e), formatted_info)

Server Traceback (most recent call last):
File "/home/opt/odoo/odoo70-prd/odoo/addons/web/session.py", line 89, in send
return openerp.netsvc.dispatch_rpc(service_name, method, args)
File "/home/opt/odoo/odoo70-prd/odoo/openerp/netsvc.py", line 296, in dispatch_rpc
result = ExportService.getService(service_name).dispatch(method, params)
File "/home/opt/odoo/odoo70-prd/odoo/openerp/service/web_services.py", line 626, in dispatch
res = fn(db, uid, _params)
File "/home/opt/odoo/odoo70-prd/odoo/openerp/osv/osv.py", line 190, in execute_kw
return self.execute(db, uid, obj, method, *args, *_kw or {})
File "/home/opt/odoo/odoo70-prd/odoo/openerp/osv/osv.py", line 132, in wrapper
return f(self, dbname, _args, *_kwargs)
File "/home/opt/odoo/odoo70-prd/odoo/openerp/osv/osv.py", line 199, in execute
res = self.execute_cr(cr, uid, obj, method, _args, *_kw)
File "/home/opt/odoo/odoo70-prd/odoo/openerp/osv/osv.py", line 187, in execute_cr
return getattr(object, method)(cr, uid, _args, *_kw)
File "/home/opt/odoo/odoo70-prd/odoo/addons/purchase/wizard/purchase_line_invoice.py", line 99, in makeInvoices
acc_id = purchase_obj._choose_account_from_po_line(cr, uid, line, context=context)
File "/home/opt/odoo/odoo70-prd/addons/Eficent/eficent-odoo-addons/account_creditor_price_difference/purchase.py", line 30, in _choose_account_from_po_line
account_id = super(purchase_order, self)._choose_account_from_po_line(cr, uid, order_line, context=context)
File "/home/opt/odoo/odoo70-prd/odoo/addons/purchase/purchase.py", line 492, in _choose_account_from_po_line
acc_id = property_obj.get(cr, uid, 'property_account_expense_categ', 'product.category', context=context).id
AttributeError: 'bool' object has no attribute 'id'