diff --git a/letsencrypt-auto-source/Dockerfile.centos6 b/letsencrypt-auto-source/Dockerfile.centos6
index 47eb48f50..92fec168b 100644
--- a/letsencrypt-auto-source/Dockerfile.centos6
+++ b/letsencrypt-auto-source/Dockerfile.centos6
@@ -11,7 +11,7 @@ RUN yum install -y python-pip sudo
 COPY ./pieces/pipstrap.py /opt
 RUN /opt/pipstrap.py
 # Pin pytest version for increased stability
-RUN pip install pytest==3.2.5
+RUN pip install pytest==3.2.5 six==1.10.0
 
 # Add an unprivileged user:
 RUN useradd --create-home --home-dir /home/lea --shell /bin/bash --groups wheel --uid 1000 lea
diff --git a/letsencrypt-auto-source/Dockerfile.precise b/letsencrypt-auto-source/Dockerfile.precise
index 71d572315..39a167c14 100644
--- a/letsencrypt-auto-source/Dockerfile.precise
+++ b/letsencrypt-auto-source/Dockerfile.precise
@@ -15,7 +15,7 @@ RUN apt-get update && \
 COPY ./pieces/pipstrap.py /opt
 RUN /opt/pipstrap.py
 # Pin pytest version for increased stability
-RUN pip install pytest==3.2.5
+RUN pip install pytest==3.2.5 six==1.10.0
 
 # Let that user sudo:
 RUN sed -i.bkp -e \
diff --git a/letsencrypt-auto-source/Dockerfile.trusty b/letsencrypt-auto-source/Dockerfile.trusty
index e0aacd118..3de88f9af 100644
--- a/letsencrypt-auto-source/Dockerfile.trusty
+++ b/letsencrypt-auto-source/Dockerfile.trusty
@@ -19,7 +19,7 @@ RUN apt-get update && \
 COPY ./pieces/pipstrap.py /opt
 RUN /opt/pipstrap.py
 # Pin pytest version for increased stability
-RUN pip install pytest==3.2.5
+RUN pip install pytest==3.2.5 six==1.10.0
 
 RUN mkdir -p /home/lea/certbot
 
diff --git a/letsencrypt-auto-source/Dockerfile.wheezy b/letsencrypt-auto-source/Dockerfile.wheezy
index 56948d22a..f4f3fea15 100644
--- a/letsencrypt-auto-source/Dockerfile.wheezy
+++ b/letsencrypt-auto-source/Dockerfile.wheezy
@@ -14,7 +14,7 @@ RUN apt-get update && \
 COPY ./pieces/pipstrap.py /opt
 RUN /opt/pipstrap.py
 # Pin pytest version for increased stability
-RUN pip install pytest==3.2.5
+RUN pip install pytest==3.2.5 six==1.10.0
 
 # Let that user sudo:
 RUN sed -i.bkp -e \
diff --git a/letsencrypt-auto-source/tests/__init__.py b/letsencrypt-auto-source/tests/__init__.py
index 45db90444..8a1613aa5 100644
--- a/letsencrypt-auto-source/tests/__init__.py
+++ b/letsencrypt-auto-source/tests/__init__.py
@@ -2,6 +2,6 @@
 
 Run these locally by saying... ::
 
-    ./build.py && docker build -t lea . && docker run --rm -t -i lea
+    ./build.py && docker build -t lea . -f Dockerfile.<distro> && docker run --rm -t -i lea
 
 """