diff --git a/tox.ini b/tox.ini
index d34aac1..1f47ca8 100644
--- a/tox.ini
+++ b/tox.ini
@@ -13,6 +13,7 @@ deps =
   haversine
   python-Levenshtein
   medallion
+  pyyaml<5.3
 commands =
   python -m pytest --cov=stix2 stix2/test/ --cov-report term-missing -W ignore::stix2.exceptions.STIXDeprecationWarning