diff --git a/tag.sh b/tag.sh
index a883084..bf63678 100755
--- a/tag.sh
+++ b/tag.sh
@@ -190,6 +190,7 @@ main() {
     fi 
 
     if [[ "$1" = "listtags" ]]; then
+        [ -z "${2:-}" ] && fail "Usage: $SCRIPT_NAME listtags filename"
         listtags "$2"
         exit 0
     fi