diff --git a/tests/requirements/base.txt b/tests/requirements/base.txt
index 08a6b3a526..d323dbbf56 100644
--- a/tests/requirements/base.txt
+++ b/tests/requirements/base.txt
@@ -5,7 +5,7 @@ geoip2
 jinja2 >= 2.9.2
 numpy
 Pillow != 5.4.0
-PyYAML
+PyYAML < 5.3
 # pylibmc/libmemcached can't be built on Windows.
 pylibmc; sys.platform != 'win32'
 python-memcached >= 1.59