diff --git a/tests/Toolkit/DomTest.php b/tests/Toolkit/DomTest.php
index fbc2ebee2f..7a66b64539 100644
--- a/tests/Toolkit/DomTest.php
+++ b/tests/Toolkit/DomTest.php
@@ -9,6 +9,7 @@
 use DOMElement;
 use Kirby\Cms\App;
 use Kirby\Exception\InvalidArgumentException;
+use PHPUnit\Framework\AssertionFailedError;
 
 /**
  * @coversDefaultClass \Kirby\Toolkit\Dom
@@ -1665,7 +1666,9 @@ public static function sanitizeProvider(): array
 				'<xml a="A" b="B"/>',
 				[
 					'attrCallback' => function (DOMAttr $attr, array $options): array {
-						// $this->assertInstanceOf(Closure::class, $options['attrCallback']);
+						if (is_a($options['attrCallback'], Closure::class) !== true) {
+							throw new AssertionFailedError('Options provided to callback are not complete or invalid');
+						}
 
 						if ($attr->nodeName === 'b') {
 							$attr->ownerElement->removeAttributeNode($attr);
@@ -1733,7 +1736,9 @@ public static function sanitizeProvider(): array
 				'<!DOCTYPE svg><xml/>',
 				[
 					'doctypeCallback' => function (DOMDocumentType $doctype, array $options): void {
-						// $this->assertInstanceOf(Closure::class, $options['doctypeCallback']);
+						if (is_a($options['doctypeCallback'], Closure::class) !== true) {
+							throw new AssertionFailedError('Options provided to callback are not complete or invalid');
+						}
 
 						throw new InvalidArgumentException('The "' . $doctype->name . '" doctype is not allowed');
 					}
@@ -1759,7 +1764,9 @@ public static function sanitizeProvider(): array
 				'<xml><a class="a">A</a><b class="b">B</b></xml>',
 				[
 					'elementCallback' => function (DOMElement $element, array $options): array {
-						// $this->assertInstanceOf(Closure::class, $options['elementCallback']);
+						if (is_a($options['elementCallback'], Closure::class) !== true) {
+							throw new AssertionFailedError('Options provided to callback are not complete or invalid');
+						}
 
 						if ($element->nodeName === 'b') {
 							Dom::remove($element);