diff --git a/setup.py b/setup.py
index 12b7832..10b4e53 100644
--- a/setup.py
+++ b/setup.py
@@ -31,6 +31,7 @@ setup(
     """,
     install_requires=[
         "requests >= 1.2.3",
+        "six >= 1.3.0",
         "numpy >= 1.8.1",
         "scikit-image >= 0.10.1"
     ],