diff --git a/tests/model_formsets_regress/tests.py b/tests/model_formsets_regress/tests.py
index 2232977481..9f7bbff05e 100644
--- a/tests/model_formsets_regress/tests.py
+++ b/tests/model_formsets_regress/tests.py
@@ -408,8 +408,8 @@ class FormfieldShouldDeleteFormTests(TestCase):
             fields = "__all__"
 
         def should_delete(self):
-            """ delete form if odd PK """
-            return self.instance.pk % 2 != 0
+            """Delete form if odd serial."""
+            return self.instance.serial % 2 != 0
 
     NormalFormset = modelformset_factory(User, form=CustomDeleteUserForm, can_delete=True)
     DeleteFormset = modelformset_factory(User, form=CustomDeleteUserForm, formset=BaseCustomDeleteModelFormSet)
@@ -492,14 +492,14 @@ class FormfieldShouldDeleteFormTests(TestCase):
         data.update(self.delete_all_ids)
         formset = self.DeleteFormset(data, queryset=User.objects.all())
 
-        # verify two were deleted
+        # Three with odd serial values were deleted.
         self.assertTrue(formset.is_valid())
         self.assertEqual(len(formset.save()), 0)
-        self.assertEqual(len(User.objects.all()), 2)
+        self.assertEqual(User.objects.count(), 1)
 
-        # verify no "odd" PKs left
-        odd_ids = [user.pk for user in User.objects.all() if user.pk % 2]
-        self.assertEqual(len(odd_ids), 0)
+        # No odd serial values left.
+        odd_serials = [user.serial for user in User.objects.all() if user.serial % 2]
+        self.assertEqual(len(odd_serials), 0)
 
 
 class RedeleteTests(TestCase):