2 files changed, 2 insertions, 4 deletions
diff --git a/src/NXP/Classes/Calculator.php b/src/NXP/Classes/Calculator.php
index 49142df..4785a8f 100644
--- a/src/NXP/Classes/Calculator.php
+++ b/src/NXP/Classes/Calculator.php
@@ -64,9 +64,7 @@ class Calculator
                     $value = $variables[$variable];
                 } elseif ($onVarNotFound) {
                     $value = call_user_func($onVarNotFound, $variable);
-                }
-
-                if (!isset($value)) {
+                } else {
                     throw new UnknownVariableException($variable);
                 }
 
diff --git a/src/NXP/MathExecutor.php b/src/NXP/MathExecutor.php
index 434246a..f45335c 100644
--- a/src/NXP/MathExecutor.php
+++ b/src/NXP/MathExecutor.php
@@ -416,7 +416,7 @@ class MathExecutor
      */
     public function getVar(string $variable)
     {
-        if (!isset($this->variables[$variable])) {
+        if (!array_key_exists($variable, $this->variables)) {
             throw new UnknownVariableException("Variable ({$variable}) not set");
         }
         return $this->variables[$variable];