diff --git a/django/forms/__init__.py b/django/forms/__init__.py
index fab7a6da7a..1e9cb2c596 100644
--- a/django/forms/__init__.py
+++ b/django/forms/__init__.py
@@ -571,7 +571,7 @@ class NullBooleanField(SelectField):
         return SelectField.render(self, data)
 
     def html2python(data):
-        return {'1': None, '2': True, '3': False}[data]
+        return {None: None, '1': None, '2': True, '3': False}[data]
     html2python = staticmethod(html2python)
 
 class SelectMultipleField(SelectField):