diff --git a/setup.py b/setup.py
index 6c866a1b1043d50ee831d08edf91ee93bd4709e5..986e4e583b1e83ceee80669c48c6c2cee54fc4fc 100644
--- a/setup.py
+++ b/setup.py
@@ -52,8 +52,9 @@ setup(name="pymbolic",
           "pymbolic.geometric_algebra",
           ],
       install_requires=[
-          'pytools>=2',
-          'pytest>=2.3',
+          "pytools>=2",
+          "pytest>=2.3",
+          "six",
           ],
 
       # 2to3 invocation