diff --git a/doc/source/conf.py b/doc/source/conf.py
index 03cc115..270ee8c 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -62,7 +62,7 @@
 #
 # 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'
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.