diff --git a/requirements/base.txt b/requirements/base.txt
index 4e735d3a21..1a16e368d0 100644
--- a/requirements/base.txt
+++ b/requirements/base.txt
@@ -3,6 +3,6 @@ msgpack-python>0.3
 PyYAML
 MarkupSafe
 requests>=1.0.0
-tornado>=4.2.1
+tornado>=4.2.1,<5.0
 # Required by Tornado to handle threads stuff.
 futures>=2.0