diff --git a/requirements.txt b/requirements.txt
index 060df56..fa7e8ea 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,6 +3,7 @@ coverage == 4.4.1
 future
 psutil >= 5.2.2
 pycodestyle >= 2.3.1
+six
 sphinx == 1.4.8
 sphinx-argparse == 0.1.15
 sphinx_rtd_theme == 0.2.4
diff --git a/setup.py b/setup.py
index f54ccd2..bb09e58 100644
--- a/setup.py
+++ b/setup.py
@@ -256,7 +256,8 @@ def long_description():
     # Dependencies.
     install_requires=[
         'psutil >= 5.2.2',
-        'future'
+        'future',
+        'six'
     ],
 
     # Tests