diff --git a/lib/GraphApplication.php b/lib/GraphApplication.php
index c8b65ba..18521a7 100644
--- a/lib/GraphApplication.php
+++ b/lib/GraphApplication.php
@@ -15,7 +15,7 @@ final class GraphApplication
     public function start(string $jsonGlob): int
     {
         $jsonFiles = glob($jsonGlob, GLOB_NOSORT);
-        \Safe\usort( $jsonFiles, function( $a, $b ) {
+        \Safe\usort( $jsonFiles, function( string $a, string $b ) {
             return \Safe\filemtime($a) - \Safe\filemtime($b);
         });