diff --git a/wagtail_modeltranslation/apps.py b/wagtail_modeltranslation/apps.py
index eff9d97..fe2275a 100644
--- a/wagtail_modeltranslation/apps.py
+++ b/wagtail_modeltranslation/apps.py
@@ -21,10 +21,10 @@ class ModeltranslationConfig(AppConfig):
         # update both the standard settings and the modeltranslation settings,
         # as we cannot guarantee the load order, and so django_modeltranslation
         # may bootstrap itself either before, or after, our ready() gets called.
-        custom_fields = getattr(settings, 'MODELTRANSLATION_CUSTOM_FIELDS', ())
+        custom_fields = getattr(settings, 'MODELTRANSLATION_CUSTOM_FIELDS', list())
         setattr(settings, 'MODELTRANSLATION_CUSTOM_FIELDS', list(set(custom_fields + wagtail_fields)))
 
-        mt_custom_fields = getattr(mt_settings, 'CUSTOM_FIELDS', ())
+        mt_custom_fields = getattr(mt_settings, 'CUSTOM_FIELDS', list())
         setattr(mt_settings, 'CUSTOM_FIELDS', list(set(mt_custom_fields + wagtail_fields)))
 
         from modeltranslation.models import handle_translation_registrations