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 59d4e87c98..137c202eba 100644
--- a/tests/roles/bootstrap-host/templates/user_variables.aio.yml.j2
+++ b/tests/roles/bootstrap-host/templates/user_variables.aio.yml.j2
@@ -96,7 +96,8 @@ apply_security_hardening: true
 
 {% if repo_build_pip_extra_indexes is defined %}
 ## Wheel mirrors for the repo_build to use
-repo_build_pip_extra_indexes: {{ repo_build_pip_extra_indexes }}
+repo_build_pip_extra_indexes:
+{{ repo_build_pip_extra_indexes | to_nice_yaml }}
 {% endif %}
 
 {% if nova_uca_enable is defined %}