diff --git a/weed-fs/src/cmd/weedvolume/fix_volume_index/fix_volume_index.go b/weed-fs/src/cmd/weedvolume/fix_volume_index/fix_volume_index.go
index 94cbc4b50..9f4c089fb 100644
--- a/weed-fs/src/cmd/weedvolume/fix_volume_index/fix_volume_index.go
+++ b/weed-fs/src/cmd/weedvolume/fix_volume_index/fix_volume_index.go
@@ -11,7 +11,7 @@ import (
 
 var (
 	dir      = flag.String("dir", "/tmp", "data directory to store files")
-	volumeId = flag.Int("volumeId", -1, "volume id")
+	volumeId = flag.Int("volumeId", -1, "a non-negative volume id. The volume should already exist in the dir. The volume index file should not exist.")
 	IsDebug  = flag.Bool("debug", false, "enable debug mode")
 
 	store *storage.Store