diff --git a/readthedocs-pip-requirements.txt b/readthedocs-pip-requirements.txt
index 59e9544..47c468b 100644
--- a/readthedocs-pip-requirements.txt
+++ b/readthedocs-pip-requirements.txt
@@ -1 +1,2 @@
 numpydoc==0.4
+contextlib2==0.4.0