Index: django/db/models/fields/__init__.py
===================================================================
--- django/db/models/fields/__init__.py	(revision 6894)
+++ django/db/models/fields/__init__.py	(working copy)
@@ -906,6 +906,11 @@
     def get_manipulator_field_objs(self):
         return [oldforms.NullBooleanField]
 
+    def formfield(self, **kwargs):
+        defaults = {'form_class': forms.NullBooleanField}
+        defaults.update(kwargs)
+        return super(NullBooleanField, self).formfield(**defaults)
+
 class PhoneNumberField(IntegerField):
     def get_manipulator_field_objs(self):
         return [oldforms.PhoneNumberField]