diff --git a/docker/Dockerfile.certbot b/docker/Dockerfile.certbot
index 34d09f5..373e570 100644
--- a/docker/Dockerfile.certbot
+++ b/docker/Dockerfile.certbot
@@ -35,7 +35,7 @@ ENV PATH="/opt/certbot/bin:$PATH"
 RUN curl -L 'https://bootstrap.pypa.io/get-pip.py' | python3
 
 RUN pip install --no-cache-dir --upgrade pyopenssl \
-	&& pip install --no-cache-dir cffi certbot cryptography \
+	&& pip install --no-cache-dir cffi certbot cryptography six \
 	&& pip install tldextract zope
 
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