Count only baskets in num_basket_additions

oscar/apps/offer/receivers.py

 from django.db.models.signals import m2m_changed, post_save
 
 from oscar.core.loading import import_module
+from oscar.apps.basket.models import AbstractBasket
 import_module('voucher.models', ['Voucher'], locals())
 import_module('order.models', ['OrderDiscount'], locals())
 
 @receiver(m2m_changed)
 def receive_basket_voucher_change(sender, **kwargs):
-    if kwargs['model'] == Voucher and kwargs['action'] == 'post_add':
+    if (kwargs['model'] == Voucher and kwargs['action'] == 'post_add' and
+        isinstance(kwargs['instance'], AbstractBasket) and kwargs['pk_set']):
         voucher_id = list(kwargs['pk_set'])[0]
         voucher = Voucher._default_manager.get(pk=voucher_id)
         voucher.num_basket_additions += 1