diff --git a/source/FluidXml/FluidInterface.php b/source/FluidXml/FluidInterface.php
index 0bfc6e4..4ab140a 100644
--- a/source/FluidXml/FluidInterface.php
+++ b/source/FluidXml/FluidInterface.php
@@ -14,7 +14,7 @@ public function html($strip = false);
         public function save($file, $strip = false);
         public function query(...$query);
         public function __invoke(...$query);
-        public function times($times, callable $fn = null);
+        public function times($times, ?callable $fn = null);
         public function each(callable $fn);
         public function map(callable $fn);
         public function filter(callable $fn);