1 files changed, 1 insertions, 1 deletions
diff --git a/src/NXP/Classes/Calculator.php b/src/NXP/Classes/Calculator.php
index bfa7015..253e70c 100644
--- a/src/NXP/Classes/Calculator.php
+++ b/src/NXP/Classes/Calculator.php
@@ -54,7 +54,7 @@ class Calculator
             }
         }
         $result = array_pop($stack);
-        if (!empty($stack)) {
+        if ($result === null || ! empty($stack)) {
             throw new IncorrectExpressionException();
         }