diff --git a/requirements.txt b/requirements.txt
index 3a3044c..2053e92 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,6 +2,7 @@
 actdiag~=2.0.0
 blockdiag~=1.5.4
 doc8~=0.8.0
+docutils==0.16
 nwdiag~=2.0.0
 recommonmark~=0.6.0
 seqdiag~=2.0.0