diff --git a/installer/roles/local_docker/tasks/standalone.yml b/installer/roles/local_docker/tasks/standalone.yml
index 763f96e00b..ad7a064921 100644
--- a/installer/roles/local_docker/tasks/standalone.yml
+++ b/installer/roles/local_docker/tasks/standalone.yml
@@ -84,8 +84,8 @@
     image: "{{ awx_web_docker_actual_image }}"
     volumes: >
       {{
-        [project_data_dir + ':/var/lib/awx/projects:z'] if project_data_dir is defined else []
-        + [ca_trust_dir + ':/etc/pki/ca-trust/source/anchors:ro'] if ca_trust_dir is defined else []
+        ([project_data_dir + ':/var/lib/awx/projects:z'] if project_data_dir is defined else [])
+        + ([ca_trust_dir + ':/etc/pki/ca-trust/source/anchors:ro'] if ca_trust_dir is defined else [])
       }}
     user: root
     ports:
@@ -127,8 +127,8 @@
     image: "{{ awx_task_docker_actual_image }}"
     volumes: >
       {{
-        [project_data_dir + ':/var/lib/awx/projects:z'] if project_data_dir is defined else []
-        + [ca_trust_dir + ':/etc/pki/ca-trust/source/anchors:ro'] if ca_trust_dir is defined else []
+        ([project_data_dir + ':/var/lib/awx/projects:z'] if project_data_dir is defined else [])
+        + ([ca_trust_dir + ':/etc/pki/ca-trust/source/anchors:ro'] if ca_trust_dir is defined else [])
       }}
     links: "{{ awx_task_container_links|list }}"
     user: root