diff --git a/setup.py b/setup.py
index 4a6b9389ec..f7be5a2955 100644
--- a/setup.py
+++ b/setup.py
@@ -29,6 +29,7 @@
     "virtualenv",
     'requests[security];python_version<"2.7"',
     'ordereddict;python_version<"2.7"',
+    'backports.weakref; python_version<"3"',
     'enum34; python_version<"3"',
     'typing; python_version<"3.5"'
 ]