diff --git a/polymorphic/query.py b/polymorphic/query.py
index eab634e..150c505 100644
--- a/polymorphic/query.py
+++ b/polymorphic/query.py
@@ -167,6 +167,15 @@ class PolymorphicQuerySet(QuerySet):
         # in the format idlist_per_model[modelclass]=[list-of-object-ids]
         idlist_per_model = defaultdict(list)
 
+        # django's automatic ".pk" field does not always work correctly for
+        # custom fields in derived objects (unclear yet who to put the blame on).
+        # We get different type(o.pk) in this case.
+        # We work around this by using the real name of the field directly
+        # for accessing the primary key of the the derived objects.
+        # We might assume that self.model._meta.pk.name gives us the name of the primary key field,
+        # but it doesn't. Therefore we use polymorphic_primary_key_name, which we set up in base.py.
+        pk_name = self.model.polymorphic_primary_key_name
+
         # - sort base_result_object ids into idlist_per_model lists, depending on their real class;
         # - also record the correct result order in "ordered_id_list"
         # - store objects that already have the correct class into "results"
@@ -198,16 +207,7 @@ class PolymorphicQuerySet(QuerySet):
                         results[base_object.pk] = transmogrify(real_concrete_class, base_object)
                     else:
                         real_concrete_class = ContentType.objects.get_for_id(real_concrete_class_id).model_class()
-                        idlist_per_model[real_concrete_class].append(base_object.pk)
-
-        # django's automatic ".pk" field does not always work correctly for
-        # custom fields in derived objects (unclear yet who to put the blame on).
-        # We get different type(o.pk) in this case.
-        # We work around this by using the real name of the field directly
-        # for accessing the primary key of the the derived objects.
-        # We might assume that self.model._meta.pk.name gives us the name of the primary key field,
-        # but it doesn't. Therefore we use polymorphic_primary_key_name, which we set up in base.py.
-        pk_name = self.model.polymorphic_primary_key_name
+                        idlist_per_model[real_concrete_class].append(getattr(base_object, pk_name))
 
         # For each model in "idlist_per_model" request its objects (the real model)
         # from the db and store them in results[].
@@ -215,7 +215,9 @@ class PolymorphicQuerySet(QuerySet):
         # Then we copy the extra() select fields from the base objects to the real objects.
         # TODO: defer(), only(): support for these would be around here
         for real_concrete_class, idlist in idlist_per_model.items():
-            real_objects = real_concrete_class.base_objects.filter(pk__in=idlist)  # use pk__in instead ####
+            real_objects = real_concrete_class.base_objects.filter(**{
+                ('%s__in' % pk_name): idlist,
+            })
             real_objects.query.select_related = self.query.select_related  # copy select related configuration to new qs
 
             for real_object in real_objects:
diff --git a/polymorphic/tests.py b/polymorphic/tests.py
index 1a28bcd..c9493fc 100644
--- a/polymorphic/tests.py
+++ b/polymorphic/tests.py
@@ -283,6 +283,12 @@ class RelatedNameClash(ShowFieldType, PolymorphicModel):
 class TestParentLinkAndRelatedName(ModelShow1_plain):
     superclass = models.OneToOneField(ModelShow1_plain, parent_link=True, related_name='related_name_subclass')
 
+class CustomPkBase(ShowFieldTypeAndContent, PolymorphicModel):
+    b = models.CharField(max_length=1)
+class CustomPkInherit(CustomPkBase):
+    custom_id = models.AutoField(primary_key=True)
+    i = models.CharField(max_length=1)
+
 
 class PolymorphicTests(TestCase):
     """
@@ -304,7 +310,6 @@ class PolymorphicTests(TestCase):
             print('DiamondXY fields 1: field_b "{0}", field_x "{1}", field_y "{2}"'.format(o1.field_b, o1.field_x, o1.field_y))
             print('DiamondXY fields 2: field_b "{0}", field_x "{1}", field_y "{2}"'.format(o2.field_b, o2.field_x, o2.field_y))
 
-
     def test_annotate_aggregate_order(self):
         # create a blog of type BlogA
         # create two blog entries in BlogA
@@ -842,6 +847,13 @@ class PolymorphicTests(TestCase):
         self.assertIsInstance(objects[0], ProxyModelA)
         self.assertIsInstance(objects[1], ProxyModelB)
 
+    def test_custom_pk(self):
+        CustomPkBase.objects.create(b='b')
+        CustomPkInherit.objects.create(b='b', i='i')
+        qs = CustomPkBase.objects.all()
+        self.assertEqual(len(qs), 2)
+        self.assertEqual(repr(qs[0]), '<CustomPkBase: id 1, b (CharField) "b">')
+        self.assertEqual(repr(qs[1]), '<CustomPkInherit: id 2, b (CharField) "b", custom_id (AutoField/pk) 1, i (CharField) "i">')
 
     def test_fix_getattribute(self):
         ### fixed issue in PolymorphicModel.__getattribute__: field name same as model name