diff --git a/conf/umount_disk.sh b/conf/umount_disk.sh
index 15e6314..9ecc0d6 100755
--- a/conf/umount_disk.sh
+++ b/conf/umount_disk.sh
@@ -1,7 +1,7 @@
 #!/bin/bash
 
-# If we're inside a container
-if systemd-detect-virt -c -q
+# If we're NOT inside a container
+if ! systemd-detect-virt -c -q
 then
     data_dir=__DATA_DIR__
     nbd=$(cat $data_dir/nbd_index)