diff --git a/src/Mustache/Parser.php b/src/Mustache/Parser.php
index 5db58244..ae35a8dd 100644
--- a/src/Mustache/Parser.php
+++ b/src/Mustache/Parser.php
@@ -72,7 +72,7 @@ public function setPragmas(array $pragmas)
      *
      * @return array Mustache Token parse tree
      */
-    private function buildTree(array &$tokens, array $parent = null)
+    private function buildTree(array &$tokens, ?array $parent = null)
     {
         $nodes = array();