diff --git a/requirements.txt b/requirements.txt
index bda5ca3e..49239f1c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,5 @@
 # Packages needed for using this library.
+pbr>=0.5.21,<1.0
 anyjson>=0.3.3
 iso8601>=0.1.4
 # Python 2->3 compatibility library.