diff --git a/doc/requirements.txt b/doc/requirements.txt
index 6da433c..0517967 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,6 +1,6 @@
 alabaster==0.7.10
 argh==0.26.2
-Babel==2.5.1
+Babel==2.9.1
 certifi==2017.11.5
 chardet==3.0.4
 CommonMark==0.5.4