diff --git a/doc/source/conf.py b/doc/source/conf.py
index 59f36ac..ac2dd71 100755
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -57,7 +57,7 @@ release = version_info.release_string()
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = 'en'
 
 add_function_parentheses = True
 add_module_names = False