diff --git a/Containers/mastercontainer/start.sh b/Containers/mastercontainer/start.sh
index 9b948776..adb91c39 100644
--- a/Containers/mastercontainer/start.sh
+++ b/Containers/mastercontainer/start.sh
@@ -43,6 +43,11 @@ elif ! mountpoint -q "/mnt/docker-aio-config"; then
     echo "Please make sure to mount the nextcloud_aio_mastercontainer docker volume into /mnt/docker-aio-config inside the container!"
     echo "If you are on TrueNas SCALE, see https://github.com/nextcloud/all-in-one#can-i-run-aio-on-truenas-scale"
     exit 1
+elif mountpoint -q /var/www/docker-aio/php/containers.json; then
+    print_red "/var/www/docker-aio/php/containers.json is a mountpoint. Cannot proceed!"
+    echo "This is a not-supported customization of the mastercontainer!"
+    echo "Please remove this bind-mount from the mastercontainer."
+    exit 1
 elif ! sudo -u www-data test -r /var/run/docker.sock; then
     echo "Trying to fix docker.sock permissions internally..."
     DOCKER_GROUP=$(stat -c '%G' /var/run/docker.sock)