diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -60,7 +60,7 @@ coreconfigitem('patch', 'fuzz',
     default=2,
 )
 coreconfigitem('ui', 'clonebundleprefers',
-    default=[],
+    default=list,
 )
 coreconfigitem('ui', 'interactive',
     default=None,
diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -455,6 +455,8 @@ class ui(object):
             if default is _unset:
                 if item is None:
                     value = default
+                elif callable(item.default):
+                    value = item.default()
                 else:
                     value = item.default
             elif item is not None: