diff --git a/lib/MathParser/Math.php b/lib/MathParser/Math.php
index 3d12899..08b31ae 100644
--- a/lib/MathParser/Math.php
+++ b/lib/MathParser/Math.php
@@ -6,7 +6,7 @@
 
 class Math
 {
-    protected $variables = [];
+    private $variables = [];
     
     /**
      * @param $string