diff --git a/optipdf b/optipdf
index 3aaa51c..d61785b 100755
--- a/optipdf
+++ b/optipdf
@@ -104,6 +104,11 @@ for ARG; do
     fi
 done
 
+if [[ ${#FILES[@]} -eq 0 ]]; then
+    USAGE >&2
+    exit 1
+fi
+
 file_size() {
     stat \
         --format='%s' \