diff --git a/ch11/chapter_11_UNDER_40/requirements.txt b/ch11/chapter_11_UNDER_40/requirements.txt
index a8b804e26..39e515a77 100644
--- a/ch11/chapter_11_UNDER_40/requirements.txt
+++ b/ch11/chapter_11_UNDER_40/requirements.txt
@@ -7,7 +7,7 @@ numpy==1.14.0
 pyparsing==2.2.0
 python-dateutil==2.6.1
 pytz==2018.3
-PyYAML==5.1
+PyYAML==5.4
 scikit-learn==0.19.1
 scipy==1.0.0
 six==1.11.0