diff --git a/etc/taskcluster/docker/base.dockerfile b/etc/taskcluster/docker/base.dockerfile
index 568741185cfd..3015c54e2843 100644
--- a/etc/taskcluster/docker/base.dockerfile
+++ b/etc/taskcluster/docker/base.dockerfile
@@ -32,5 +32,5 @@ RUN \
         curl \
     && \
     # Running mach
-    pip install virtualenv
+    pip install "virtualenv<20"