diff --git a/pylons_app/model/forms.py b/pylons_app/model/forms.py
--- a/pylons_app/model/forms.py
+++ b/pylons_app/model/forms.py
@@ -332,6 +332,7 @@ def DefaultPermissionsForm(perms_choices
     class _DefaultPermissionsForm(formencode.Schema):
         allow_extra_fields = True
         filter_extra_fields = True
+        overwrite_default = OneOf(['true', 'false'], if_missing='false')
         default_perm = OneOf(perms_choices)
         default_register = OneOf(register_choices)
         default_create = OneOf(create_choices)