diff --git a/Route.php b/Route.php
index 6e38263..ecf679f 100644
--- a/Route.php
+++ b/Route.php
@@ -103,8 +103,11 @@ abstract class Route
         );
     }
 
-    static function group(string $prefix = '', callable $callback)
+    static function group(string $prefix = '', ?callable $callback = null)
     {
+		if ($callback === null) {
+			throw new InvalidArgumentException('Argument $callback must be of type callable.');
+		}
         if (!str_starts_with(
             self::getRequestPath(),
             self::$prefix . $prefix