diff --git a/mkrel.sh b/mkrel.sh
index c93afa7..cb07229 100755
--- a/mkrel.sh
+++ b/mkrel.sh
@@ -27,6 +27,11 @@ windows/amd64"
 tool="$1"
 version="$2"
 
+if test -z "$version"; then
+  echo "Usage: $0 <tool name> <release version>"
+  exit 1
+fi
+
 rm -rf releases
 mkdir -p releases