diff --git a/playbooks/roles/forum/defaults/main.yml b/playbooks/roles/forum/defaults/main.yml
index 41c174daee0..84c0738af06 100644
--- a/playbooks/roles/forum/defaults/main.yml
+++ b/playbooks/roles/forum/defaults/main.yml
@@ -19,7 +19,7 @@ FORUM_MONGO_HOSTS:
 FORUM_MONGO_TAGS: !!null
 FORUM_MONGO_PORT: "27017"
 FORUM_MONGO_DATABASE: "cs_comments_service"
-FORUM_MONGO_AUTH_DB: !!null
+FORUM_MONGO_AUTH_DB: "{{ FORUM_MONGO_DATABASE }}"
 FORUM_MONGO_USE_SSL: false
 FORUM_MONGO_URL: "mongodb://{{ FORUM_MONGO_USER }}:{{ FORUM_MONGO_PASSWORD }}@{%- for host in FORUM_MONGO_HOSTS -%}{{ host }}:{{ FORUM_MONGO_PORT }}{%- if not loop.last -%},{%- endif -%}{%- endfor -%}/{{ FORUM_MONGO_DATABASE }}{%- if FORUM_MONGO_TAGS -%}?tags={{ FORUM_MONGO_TAGS }}{%- endif -%}"
 FORUM_SINATRA_ENV: "development"