diff --git a/doc/conf.py.in b/doc/conf.py.in
index 684097b..587d3b1 100644
--- a/doc/conf.py.in
+++ b/doc/conf.py.in
@@ -84,7 +84,7 @@ master_doc = 'index'
 #
 # 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'
 
 # The name of the Pygments (syntax highlighting) style to use.
 pygments_style = 'sphinx'