diff --git a/docker-compose.yml b/docker-compose.yml
index 60f85f1..38f311d 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -24,7 +24,7 @@ services:
       MONGO_INITDB_ROOT_PASSWORD: ${PYTHON_MONGO_PASSWORD:?error}
       MONGO_INITDB_DATABASE: ${PYTHON_MONGO_DATABASE:-transparenzregister}
     ports:
-    - {MONGO_EXTERNAL_PORT:-27018}:27017
+    - ${MONGO_EXTERNAL_PORT:-27018}:27017
 
     volumes:
     - /iscsi/truenas/transparenzregister/mongo_data:/data/db