diff --git a/acme/setup.py b/acme/setup.py
index dad845c04..d48e9e89f 100644
--- a/acme/setup.py
+++ b/acme/setup.py
@@ -20,7 +20,7 @@ install_requires = [
     # For pkg_resources. >=1.0 so pip resolves it to a version cryptography
     # will tolerate; see #2599:
     'setuptools>=1.0',
-    'six',
+    'six>=1.9.0',  # needed for python_2_unicode_compatible
 ]
 
 # env markers cause problems with older pip and setuptools