diff --git a/Dockerfile b/Dockerfile
index 546e28d69d..a9892031b8 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -3,6 +3,7 @@ FROM debian:wheezy
 RUN set -ex; \
     apt-get update -qq; \
     apt-get install -y \
+        locales \
         gcc \
         make \
         zlib1g \
@@ -61,6 +62,10 @@ RUN set -ex; \
     rm -rf pip-7.0.1; \
     rm pip-7.0.1.tar.gz
 
+# Python3 requires a valid locale
+RUN echo "en_US.UTF-8 UTF-8" > /etc/locale.gen && locale-gen
+ENV LANG en_US.UTF-8
+
 ENV ALL_DOCKER_VERSIONS 1.7.1 1.8.1
 
 RUN set -ex; \
diff --git a/requirements-dev.txt b/requirements-dev.txt
index c8a694ab27..adb4387df2 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -1,6 +1,6 @@
+coverage==3.7.1
 flake8==2.3.0
 git+https://github.com/pyinstaller/pyinstaller.git@12e40471c77f588ea5be352f7219c873ddaae056#egg=pyinstaller
 mock >= 1.0.1
 nose==1.3.4
 pep8==1.6.1
-coverage==3.7.1