1 files changed, 1 insertions, 1 deletions
diff --git a/src/NXP/Classes/Operator.php b/src/NXP/Classes/Operator.php
index 53550a9..c3762ea 100644
--- a/src/NXP/Classes/Operator.php
+++ b/src/NXP/Classes/Operator.php
@@ -52,7 +52,7 @@ class Operator
         $this->places = $reflection->getNumberOfParameters();
     }
 
-    public function execute(&$stack)
+    public function execute(array &$stack) : Token
     {
         if (count($stack) < $this->places) {
             throw new IncorrectExpressionException();