diff --git a/PVE/Storage/Plugin.pm b/PVE/Storage/Plugin.pm
index aefc7f8..8ae78e9 100644
--- a/PVE/Storage/Plugin.pm
+++ b/PVE/Storage/Plugin.pm
@@ -528,13 +528,15 @@ sub create_base {
 my $get_vm_disk_number = sub {
     my ($disk_name, $scfg, $vmid, $suffix) = @_;
 
+    my $disk_regex = qr/(vm|base)-$vmid-disk-(\d+)$suffix/;
+
     my $type = $scfg->{type};
     my $def = { %{$defaultData->{plugindata}->{$type}} };
-    my $valid_formats = $def->{format}[0];
 
-    my $disk_regex = qr/(vm|base)-$vmid-disk-(\d+)$suffix/;
-    $disk_regex = qr/(vm|base|subvol|basevol)-$vmid-disk-(\d+)/
-	if $valid_formats->{subvol};
+    my $valid = $def->{format}[0];
+    if ($valid->{subvol}) {
+	$disk_regex = qr/(vm|base|subvol|basevol)-$vmid-disk-(\d+)/;
+    }
 
     if ($disk_name =~ m/$disk_regex/) {
 	return $2;