diff --git a/doc/conf.py b/doc/conf.py
index d72fa0e0..19abd646 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -56,7 +56,7 @@
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
-language = None
+language = 'en'
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used: