diff --git a/django/db/models/base.py b/django/db/models/base.py
index 3a3fbf16ac4d7..13ff7e8f35b15 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -538,7 +538,7 @@ def _collect_sub_objects(self, seen_objs, parent=None, nullable=False):
         # traversing to the most remote parent classes -- those with no parents
         # themselves -- and then adding those instances to the collection. That
         # will include all the child instances down to "self".
-        parent_stack = self._meta.parents.values()
+        parent_stack = [p for p in self._meta.parents.values() if p is not None]
         while parent_stack:
             link = parent_stack.pop()
             parent_obj = getattr(self, link.name)
diff --git a/tests/modeltests/proxy_models/models.py b/tests/modeltests/proxy_models/models.py
index baa58cba03395..4b3f7d925d2b6 100644
--- a/tests/modeltests/proxy_models/models.py
+++ b/tests/modeltests/proxy_models/models.py
@@ -276,6 +276,15 @@ class Meta:
 >>> UserProxyProxy.objects.all()
 [<UserProxyProxy: Bruce>]
 
+# Proxy objects can be deleted
+>>> u2 = UserProxy.objects.create(name='George')
+>>> UserProxy.objects.all()
+[<UserProxy: Bruce>, <UserProxy: George>]
+>>> u2.delete()
+>>> UserProxy.objects.all()
+[<UserProxy: Bruce>]
+
+
 # We can still use `select_related()` to include related models in our querysets.
 >>> country = Country.objects.create(name='Australia')
 >>> state = State.objects.create(name='New South Wales', country=country)