diff --git a/swarm-proxy-letsencrypt/Dockerfile b/swarm-proxy-letsencrypt/Dockerfile
index 88576d8875ed82d7302f88153f92d6c3126cdb9d..ef81a88e2bc5c8768de4be42242de9703250ed54 100644
--- a/swarm-proxy-letsencrypt/Dockerfile
+++ b/swarm-proxy-letsencrypt/Dockerfile
@@ -26,7 +26,8 @@ RUN sync && \
     python3 -m pip install -U \
       pip \
       setuptools \
-      wheel && \
+      wheel \
+      six && \
     python3 -m pip install . && \
     cd / && \
     rm -rf /src && \