diff --git a/playbooks/os-cinder-install.yml b/playbooks/os-cinder-install.yml
index c879673ce1..b1436d2cdd 100644
--- a/playbooks/os-cinder-install.yml
+++ b/playbooks/os-cinder-install.yml
@@ -25,7 +25,7 @@
   include: common-playbooks/cinder.yml
   vars:
     cinder_hosts: "cinder_volume:!cinder_scheduler:!cinder_api"
-    cinder_serial: "{{ cinder_backend_serial | default('1', '100%') }}"
+    cinder_serial: "{{ cinder_backend_serial | default(['1', '100%']) }}"