diff --git a/doc/conf.py b/doc/conf.py
index 73590ed5..2ef20afa 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -90,7 +90,7 @@ def napoleon_transform(comment):
 #
 # 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.