diff --git a/src/NXP/MathExecutor.php b/src/NXP/MathExecutor.php
index a113275..ab550fa 100644
--- a/src/NXP/MathExecutor.php
+++ b/src/NXP/MathExecutor.php
@@ -31,27 +31,32 @@ class MathExecutor
      *
      * @var array<string, float|string>
      */
-    private $variables = [];
+    protected $variables = [];
 
     /**
      * @var callable|null
      */
-    private $onVarNotFound = null;
+    protected $onVarNotFound = null;
+
+    /**
+     * @var callable|null
+     */
+    protected $onVarValidation = null;
 
     /**
      * @var Operator[]
      */
-    private $operators = [];
+    protected $operators = [];
 
     /**
      * @var array<string, CustomFunction>
      */
-    private $functions = [];
+    protected $functions = [];
 
     /**
      * @var array<string, Token[]>
      */
-    private $cache = [];
+    protected $cache = [];
 
     /**
      * Base math operators
@@ -74,6 +79,8 @@ protected function addDefaults(): void
         foreach ($this->defaultFunctions() as $name => $callable) {
             $this->addFunction($name, $callable);
         }
+
+        $this->onVarValidation = [$this, 'defaultVarValidation'];
         $this->variables = $this->defaultVars();
     }
 
@@ -507,23 +514,37 @@ public function getVar(string $variable)
     }
 
     /**
-     * Add variable to executor
+     * Add variable to executor. To set a custom validator use setVarValidationHandler.
      *
      * @param  string $variable
-     * @param  int|float $value
+     * @param  $value
      * @return MathExecutor
+     * @throws MathExecutorException if the value is invalid based on the default or custom validator
      */
     public function setVar(string $variable, $value): self
     {
-        if (!is_scalar($value) && $value !== null) {
-            $type = gettype($value);
-            throw new MathExecutorException("Variable ({$variable}) type ({$type}) is not scalar");
+        if ($this->onVarValidation) {
+            call_user_func($this->onVarValidation, $variable, $value);
         }
 
         $this->variables[$variable] = $value;
         return $this;
     }
 
+    /**
+     * Default variable validation, ensures that the value is a scalar.
+     * @param string $variable
+     * @param $value
+     * @throws MathExecutorException if the value is not a scalar
+     */
+    protected function defaultVarValidation(string $variable, $value): void
+    {
+        if (!is_scalar($value) && $value !== null) {
+            $type = gettype($value);
+            throw new MathExecutorException("Variable ({$variable}) type ({$type}) is not scalar");
+        }
+    }
+
     /**
      * Test to see if a variable exists
      *
@@ -567,6 +588,21 @@ public function setVarNotFoundHandler(callable $handler): self
         return $this;
     }
 
+    /**
+     * Define a validation method that will be invoked when a variable is set using setVar.
+     * The first parameter will be the variable name, and the second will be the variable value.
+     * Set to null to disable validation.
+     *
+     * @param ?callable $handler throws a MathExecutorException in case of an invalid variable
+     *
+     * @return MathExecutor
+     */
+    public function setVarValidationHandler(?callable $handler): self
+    {
+        $this->onVarValidation = $handler;
+        return $this;
+    }
+
     /**
      * Remove variable from executor
      *
diff --git a/tests/MathTest.php b/tests/MathTest.php
index 3eca351..14de2bf 100644
--- a/tests/MathTest.php
+++ b/tests/MathTest.php
@@ -589,6 +589,21 @@ public function testSetVarsDoesNotAcceptResource()
         $calculator->setVar('resource', tmpfile());
     }
 
+    public function testSetCustomVarValidator()
+    {
+        $calculator = new MathExecutor();
+        $calculator->setVarValidationHandler(function ($name, $variable) {
+            if ($name === 'invalidVar' && $variable === 'invalid') {
+                throw new MathExecutorException("Invalid variable");
+            }
+        });
+
+        $calculator->setVar('valid', $this);
+
+        $this->expectException(MathExecutorException::class);
+        $calculator->setVar('invalidVar', 'invalid');
+    }
+
     public function testVarExists()
     {
         $calculator = new MathExecutor();