diff --git a/stretch-amd64/mkrescue b/stretch-amd64/mkrescue
index 93a0674..bfcc5fe 100755
--- a/stretch-amd64/mkrescue
+++ b/stretch-amd64/mkrescue
@@ -2,6 +2,11 @@
 
 TARGET=$1
 
+if [ -z "${TARGET}" ]; then
+    echo "Usage: $0 <work_dir>"
+    exit 255
+fi
+
 mkdir ${TARGET}
 mkdir ${TARGET}/ramdisk
 mkdir ${TARGET}/firmware