diff -r 19d81cc213d7 Tools/clinic/clinic.py
--- a/Tools/clinic/clinic.py	Fri Jan 31 22:03:12 2014 -0800
+++ b/Tools/clinic/clinic.py	Sat Feb 01 23:26:05 2014 +0800
@@ -2030,7 +2030,7 @@
     # automatically add converter for default format unit
     # (but without stomping on the existing one if it's already
     # set, in case you subclass)
-    if ((cls.format_unit != 'O&') and
+    if ((cls.format_unit not in ('O&', '')) and
         (cls.format_unit not in legacy_converters)):
         legacy_converters[cls.format_unit] = cls
         if cls.format_unit: