Fixes the function to get ancestors content types.

wagtail/contrib/postgres_search/utils.py

@@ -68,11 +68,12 @@ def get_content_type_pk(model):
 
 def get_ancestors_content_types_pks(model):
     """
-    Returns content types ids for the descendants of this model, excluding it.
+    Returns content types ids for the ancestors of this model, excluding it.
     """
     from django.contrib.contenttypes.models import ContentType
     return [ct.pk for ct in
-            ContentType.objects.get_for_models(*model._meta.parents).values()]
+            ContentType.objects.get_for_models(*model._meta.get_parent_list())
+            .values()]
 
 
 def get_descendants_content_types_pks(model):