Index: __init__.py
===================================================================
--- __init__.py	(revision 5312)
+++ __init__.py	(working copy)
@@ -405,6 +405,7 @@
         if value in (True, False): return value
         if value in ('t', 'True', '1'): return True
         if value in ('f', 'False', '0'): return False
+        if value is None and self.null == True: return None
         raise validators.ValidationError, gettext("This value must be either True or False.")
 
     def get_manipulator_field_objs(self):