diff --git a/doc/requirements.txt b/doc/requirements.txt
index eccc937..5bc71f0 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,2 +1,3 @@
 sphinx>=2.0.0,!=2.1.0 # BSD
 openstackdocstheme>=2.2.1 # Apache-2.0
+PyYAML>=3.1.0 # MIT