diff --git a/Explore an HCA Data Set in Scanpy (Nov 2018)/requirements.txt b/Explore an HCA Data Set in Scanpy (Nov 2018)/requirements.txt
index 04a5f33..edb868e 100644
--- a/Explore an HCA Data Set in Scanpy (Nov 2018)/requirements.txt	
+++ b/Explore an HCA Data Set in Scanpy (Nov 2018)/requirements.txt	
@@ -6,3 +6,4 @@ numpy
 pandas
 python-igraph
 scanpy>=1.3.2
+tornado>=6.4.2 # not directly required, pinned by Snyk to avoid a vulnerability