diff --git a/tests/roles/bootstrap-host/templates/user_variables.aio.yml.j2 b/tests/roles/bootstrap-host/templates/user_variables.aio.yml.j2
index fda6b3e751..2d459762f4 100644
--- a/tests/roles/bootstrap-host/templates/user_variables.aio.yml.j2
+++ b/tests/roles/bootstrap-host/templates/user_variables.aio.yml.j2
@@ -163,7 +163,8 @@ galera_client_apt_repo_url: {{ galera_repo_url }}
 
 {% if lxc_image_cache_server is defined %}
 ## images.linuxcontainers.org reverse proxy
-lxc_image_cache_server_mirrors: {{ ['http://' + lxc_image_cache_server] }}
+lxc_image_cache_server_mirrors:
+  - "http://{{ lxc_image_cache_server }}"
 {% endif %}
 
 {% if cache_timeout is defined %}