diff --git a/requirements.txt b/requirements.txt
index ee153b9..d897794 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,3 +1,4 @@
-pyyaml
-icalendar
+pbr>=0.5.21,<1.0
 argparse
+icalendar
+pyyaml