diff --git a/polymorphic/query.py b/polymorphic/query.py
index 35d091a..394c101 100644
--- a/polymorphic/query.py
+++ b/polymorphic/query.py
@@ -154,6 +154,15 @@ 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"
@@ -185,16 +194,7 @@ 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[].
@@ -202,7 +202,9 @@ 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: