diff --git a/roles/matrix-nginx-proxy/tasks/validate_config.yml b/roles/matrix-nginx-proxy/tasks/validate_config.yml
index 9661ae5e9..39a2dfe59 100644
--- a/roles/matrix-nginx-proxy/tasks/validate_config.yml
+++ b/roles/matrix-nginx-proxy/tasks/validate_config.yml
@@ -43,5 +43,7 @@
         msg: "The `{{ item }}` variable must be defined and have a non-null value"
       with_items:
         - "matrix_ssl_lets_encrypt_support_email"
+        - "matrix_nginx_proxy_proxy_synapse_federation_api_addr_sans_container"
+        - "matrix_nginx_proxy_proxy_synapse_federation_api_addr_with_container"
       when: "vars[item] == '' or vars[item] is none"
   when: "matrix_ssl_retrieval_method == 'lets-encrypt'"