diff --git a/README.md b/README.md
index 54d38c6..cf8953b 100644
--- a/README.md
+++ b/README.md
@@ -237,7 +237,7 @@ abstract class Handler {
     protected array $levels;
 
     protected bool $_bubbles = true;
-    protected string|\Closure|null $_messageTransform = null;
+    protected callable $_messageTransform = null;
     protected string $_timeFormat = 'M d H:i:s';
 
     public function __construct(array $levels = [ 0, 1, 2, 3, 4, 5, 6, 7 ], array $options = []);
diff --git a/lib/Logger/Handler.php b/lib/Logger/Handler.php
index e09a2e9..0ddd6b0 100644
--- a/lib/Logger/Handler.php
+++ b/lib/Logger/Handler.php
@@ -12,7 +12,7 @@ abstract class Handler {
     protected array $levels;
 
     protected bool $_bubbles = true;
-    protected string|\Closure|null $_messageTransform = null;
+    protected $_messageTransform = null;
     protected string $_timeFormat = 'M d H:i:s';
 
 
@@ -20,14 +20,8 @@ abstract class Handler {
     public function __construct(array $levels = [ 0, 1, 2, 3, 4, 5, 6, 7 ], array $options = []) {
         $this->levels = $this->verifyLevels($levels);
 
-        $class = get_class($this);
         foreach ($options as $key => $value) {
-            $name = "_$key";
-            if (!property_exists($class, $name)) {
-                trigger_error(sprintf('Undefined option in %s: %s', $class, $key), \E_USER_WARNING);
-                continue;
-            }
-            $this->$name = $value;
+            $this->setOption($key, $value);
         }
     }
 
@@ -59,6 +53,10 @@ abstract class Handler {
             return;
         }
 
+        if ($name === 'messageTransform' && !is_callable($value)) {
+            throw new TypeError(sprintf('Value of messageTransform option must be callable'));
+        }
+
         $name = "_$name";
         $this->$name = $value;
     }
diff --git a/lib/Logger/TypeError.php b/lib/Logger/TypeError.php
new file mode 100644
index 0000000..a18ec0b
--- /dev/null
+++ b/lib/Logger/TypeError.php
@@ -0,0 +1,11 @@
+<?php
+/**
+ * @license MIT
+ * Copyright 2022 Dustin Wilson, et al.
+ * See LICENSE and AUTHORS files for details
+ */
+
+declare(strict_types=1);
+namespace MensBeam\Logger;
+
+class TypeError extends \TypeError {}
\ No newline at end of file
diff --git a/test b/test
index b914b14..2a18b7c 100755
--- a/test
+++ b/test
@@ -1,5 +1,11 @@
 #!/usr/bin/env php
 <?php
+/**
+ * @license MIT
+ * Copyright 2022 Dustin Wilson, et al.
+ * See LICENSE and AUTHORS files for details
+ */
+
 $dir = ini_get('extension_dir');
 $php = escapeshellarg(\PHP_BINARY);
 $code = escapeshellarg(__DIR__ . '/lib');
diff --git a/tests/Bootstrap.php b/tests/Bootstrap.php
index 36bb5e6..0d78d88 100644
--- a/tests/Bootstrap.php
+++ b/tests/Bootstrap.php
@@ -6,6 +6,5 @@ ini_set('memory_limit', '2G');
 ini_set('zend.assertions', '1');
 ini_set('assert.exception', 'true');
 error_reporting(\E_ALL);
-require_once dirname(__DIR__) . '/vendor/autoload.php';
-
-define('CWD', getcwd());
\ No newline at end of file
+define('CWD', dirname(__DIR__));
+require_once CWD . '/vendor/autoload.php';
\ No newline at end of file
diff --git a/tests/cases/TestHandler.php b/tests/cases/TestHandler.php
index 0ead2f4..6fba728 100644
--- a/tests/cases/TestHandler.php
+++ b/tests/cases/TestHandler.php
@@ -13,7 +13,8 @@ use MensBeam\Logger\{
     InvalidArgumentException,
     Level,
     RangeException,
-    StreamHandler
+    StreamHandler,
+    TypeError
 };
 
 
@@ -99,6 +100,14 @@ class TestHandler extends ErrorHandlingTestCase {
 
     public static function provideFatalErrorTests(): iterable {
         $iterable = [
+            [
+                TypeError::class,
+                0,
+                'Value of messageTransform option must be callable',
+                function (Handler $h): void {
+                    $h->setOption('messageTransform', 42);
+                }
+            ],
             [
                 InvalidArgumentException::class,
                 0,