diff --git a/PVE/Storage/Plugin.pm b/PVE/Storage/Plugin.pm
index 002b265..8089302 100644
--- a/PVE/Storage/Plugin.pm
+++ b/PVE/Storage/Plugin.pm
@@ -597,7 +597,7 @@ sub free_image {
 
     my $path = $class->filesystem_path($scfg, $volname);
 
-    if ($format eq 'subvol') {
+    if (defined($format) && ($format eq 'subvol')) {
 	File::Path::remove_tree($path);
     } else {