diff --git a/sphinx/builders/gettext.py b/sphinx/builders/gettext.py
index 07e462c40..b07b8efe7 100644
--- a/sphinx/builders/gettext.py
+++ b/sphinx/builders/gettext.py
@@ -112,7 +112,8 @@ class I18nBuilder(Builder):
     """
     name = 'i18n'
     versioning_method = 'text'
-    versioning_compare = None  # be set by `gettext_uuid`
+    versioning_compare = None   # type: bool
+                                # be set by `gettext_uuid`
     use_message_catalog = False
 
     def init(self):