diff --git a/requirements.txt b/requirements.txt
index 0616d66..70df204 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -20,4 +20,4 @@ six>=1.11.0
 tqdm>=4.19.5
 typing>=3.6.4
 urllib3>=1.21.1
-virtualenv>=16
+virtualenv>=16,<20