diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7dfd3f9..7b87922 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+1.4.0
+=====
+
+* (feature) Add `ValidJsonVerifier`, that checks, that every normalizer correctly generated JSON-compatible values.
+
+
 1.3.3
 =====
 
diff --git a/config/services.yaml b/config/services.yaml
index 31cd779..20fda9b 100644
--- a/config/services.yaml
+++ b/config/services.yaml
@@ -11,3 +11,4 @@ services:
 
     Torr\SimpleNormalizer\Normalizer\SimpleNormalizer:
         $objectNormalizers: !tagged_locator {tag: 'torr.normalizer.simple-object-normalizer', default_index_method: 'getNormalizedType'}
+        $isDebug: '%kernel.debug%'
diff --git a/src/Exception/IncompleteNormalizationException.php b/src/Exception/IncompleteNormalizationException.php
new file mode 100644
index 0000000..9735e2a
--- /dev/null
+++ b/src/Exception/IncompleteNormalizationException.php
@@ -0,0 +1,10 @@
+<?php declare(strict_types=1);
+
+namespace Torr\SimpleNormalizer\Exception;
+
+/**
+ * @final
+ */
+class IncompleteNormalizationException extends \RuntimeException implements NormalizerExceptionInterface
+{
+}
diff --git a/src/Normalizer/SimpleNormalizer.php b/src/Normalizer/SimpleNormalizer.php
index ac39310..2e71627 100644
--- a/src/Normalizer/SimpleNormalizer.php
+++ b/src/Normalizer/SimpleNormalizer.php
@@ -7,12 +7,17 @@
 use Symfony\Component\DependencyInjection\ServiceLocator;
 use Torr\SimpleNormalizer\Exception\ObjectTypeNotSupportedException;
 use Torr\SimpleNormalizer\Exception\UnsupportedTypeException;
+use Torr\SimpleNormalizer\Normalizer\Validator\ValidJsonVerifier;
 
 /**
  * The normalizer to use in your app.
  *
  * Can't be readonly, as it needs to be mock-able.
  *
+ * The verifier is done on the top-level of every method (instead of at the point where the invalid values could occur
+ * = the object normalizers), as this way we can provide a full path to the invalid element in the JSON.
+ *
+ * @readonly
  * @final
  */
 class SimpleNormalizer
@@ -22,11 +27,61 @@ class SimpleNormalizer
 	 */
 	public function __construct (
 		private readonly ServiceLocator $objectNormalizers,
+		private readonly bool $isDebug = false,
+		private readonly ?ValidJsonVerifier $validJsonVerifier = null,
 	) {}
 
 	/**
 	 */
 	public function normalize (mixed $value, array $context = []) : mixed
+	{
+		$normalizedValue = $this->recursiveNormalize($value, $context);
+
+		if ($this->isDebug)
+		{
+			$this->validJsonVerifier?->ensureValidOnlyJsonTypes($normalizedValue);
+		}
+
+		return $normalizedValue;
+	}
+	/**
+	 */
+	public function normalizeArray (array $array, array $context = []) : array
+	{
+		$normalizedValue = $this->recursiveNormalizeArray($array, $context);
+
+		if ($this->isDebug)
+		{
+			$this->validJsonVerifier?->ensureValidOnlyJsonTypes($normalizedValue);
+		}
+
+		return $normalizedValue;
+	}
+
+	/**
+	 * Normalizes a map of values.
+	 * Will JSON-encode to `{}` when empty.
+	 */
+	public function normalizeMap (array $array, array $context = []) : array|\stdClass
+	{
+		// return stdClass if the array is empty here, as it will be automatically normalized to `{}` in JSON.
+		$normalizedValue = $this->recursiveNormalizeArray($array, $context) ?: new \stdClass();
+
+		if ($this->isDebug)
+		{
+			$this->validJsonVerifier?->ensureValidOnlyJsonTypes($normalizedValue);
+		}
+
+		return $normalizedValue;
+	}
+
+
+
+	/**
+	 * The actual normalize logic, that recursively normalizes the value.
+	 * It must never call one of the public methods above and just normalizes the value.
+	 */
+	private function recursiveNormalize (mixed $value, array $context = []) : mixed
 	{
 		if (null === $value || \is_scalar($value))
 		{
@@ -35,7 +90,7 @@ public function normalize (mixed $value, array $context = []) : mixed
 
 		if (\is_array($value))
 		{
-			return $this->normalizeArray($value, $context);
+			return $this->recursiveNormalizeArray($value, $context);
 		}
 
 		if (\is_object($value))
@@ -77,15 +132,17 @@ public function normalize (mixed $value, array $context = []) : mixed
 	}
 
 	/**
+	 * The actual customized normalization logic for arrays, that recursively normalizes the value.
+	 * It must never call one of the public methods above and just normalizes the value.
 	 */
-	public function normalizeArray (array $array, array $context = []) : array
+	private function recursiveNormalizeArray (array $array, array $context = []) : array
 	{
 		$result = [];
 		$isList = array_is_list($array);
 
 		foreach ($array as $key => $value)
 		{
-			$normalized = $this->normalize($value, $context);
+			$normalized = $this->recursiveNormalize($value, $context);
 
 			// if the array was a list and the normalized value is null, just filter it out
 			if ($isList && null === $normalized)
@@ -107,14 +164,4 @@ public function normalizeArray (array $array, array $context = []) : array
 
 		return $result;
 	}
-
-	/**
-	 * Normalizes a map of values.
-	 * Will JSON-encode to `{}` when empty.
-	 */
-	public function normalizeMap (array $array, array $context = []) : array|\stdClass
-	{
-		// return stdClass if the array is empty here, as it will be automatically normalized to `{}` in JSON.
-		return $this->normalizeArray($array, $context) ?: new \stdClass();
-	}
 }
diff --git a/src/Normalizer/Validator/InvalidJsonElement.php b/src/Normalizer/Validator/InvalidJsonElement.php
new file mode 100644
index 0000000..04c3d0c
--- /dev/null
+++ b/src/Normalizer/Validator/InvalidJsonElement.php
@@ -0,0 +1,16 @@
+<?php declare(strict_types=1);
+
+namespace Torr\SimpleNormalizer\Normalizer\Validator;
+
+/**
+ * @final
+ */
+readonly class InvalidJsonElement
+{
+	/**
+	 */
+	public function __construct (
+		public mixed $value,
+		public array $path,
+	) {}
+}
diff --git a/src/Normalizer/Validator/ValidJsonVerifier.php b/src/Normalizer/Validator/ValidJsonVerifier.php
new file mode 100644
index 0000000..1b4c346
--- /dev/null
+++ b/src/Normalizer/Validator/ValidJsonVerifier.php
@@ -0,0 +1,76 @@
+<?php declare(strict_types=1);
+
+namespace Torr\SimpleNormalizer\Normalizer\Validator;
+
+use Torr\SimpleNormalizer\Exception\IncompleteNormalizationException;
+
+/**
+ * Helper to verify that a given value is valid JSON (scalars, lists or empty objects)
+ *
+ * @final
+ */
+class ValidJsonVerifier
+{
+	/**
+	 * Ensures that only valid JSON types are present in the value, that means scalars, arrays and empty objects.
+	 */
+	public function ensureValidOnlyJsonTypes (mixed $value) : void
+	{
+		$invalidElement = $this->findInvalidJsonElement($value);
+
+		if (null !== $invalidElement)
+		{
+			throw new IncompleteNormalizationException(
+				sprintf(
+					"Found a JSON-incompatible value when normalizing. Found '%s' at path '%s', but expected only scalars, arrays and empty objects.",
+					get_debug_type($invalidElement->value),
+					implode(".", $invalidElement->path),
+				),
+			);
+		}
+	}
+
+
+	/**
+	 * Searches through the value and looks for anything that isn't valid JSON
+	 * (scalars, arrays or empty objects).
+	 *
+	 * @return InvalidJsonElement|null Returns null if everything is valid, otherwise the invalid value.
+	 */
+	private function findInvalidJsonElement (mixed $value, array $path = ["$"]) : ?InvalidJsonElement
+	{
+		// scalars are always valid
+		if (null === $value || \is_scalar($value))
+		{
+			return null;
+		}
+
+		// only empty stdClass objects are allowed (as they are used to serialize to `{}`)
+		if (is_object($value))
+		{
+			return $value instanceof \stdClass && [] === get_object_vars($value)
+				? null
+				: new InvalidJsonElement($value, $path);
+		}
+
+		if (is_array($value))
+		{
+			foreach ($value as $key => $item)
+			{
+				$invalidItem = $this->findInvalidJsonElement(
+					$item,
+					[...$path, $key],
+				);
+
+				if (null !== $invalidItem)
+				{
+					return $invalidItem;
+				}
+			}
+
+			return null;
+		}
+
+		return new InvalidJsonElement($value, $path);
+	}
+}
diff --git a/tests/Normalizer/SimpleNormalizerTest.php b/tests/Normalizer/SimpleNormalizerTest.php
new file mode 100644
index 0000000..bc35564
--- /dev/null
+++ b/tests/Normalizer/SimpleNormalizerTest.php
@@ -0,0 +1,150 @@
+<?php declare(strict_types=1);
+
+namespace Tests\Torr\SimpleNormalizer\Normalizer;
+
+use Symfony\Component\DependencyInjection\ServiceLocator;
+use Tests\Torr\SimpleNormalizer\Fixture\DummyVO;
+use Torr\SimpleNormalizer\Exception\IncompleteNormalizationException;
+use Torr\SimpleNormalizer\Normalizer\SimpleNormalizer;
+use PHPUnit\Framework\TestCase;
+use Torr\SimpleNormalizer\Normalizer\SimpleObjectNormalizerInterface;
+use Torr\SimpleNormalizer\Normalizer\Validator\ValidJsonVerifier;
+
+class SimpleNormalizerTest extends TestCase
+{
+	/**
+	 *
+	 */
+	public static function provideJsonVerifierEnabled () : iterable
+	{
+		yield "normalize" => [
+			static fn (SimpleNormalizer $normalizer, mixed $value) => $normalizer->normalize($value),
+		];
+
+		yield "normalizeArray" => [
+			static fn (SimpleNormalizer $normalizer, mixed $value) => $normalizer->normalizeArray($value),
+		];
+
+		yield "normalizeMap" => [
+			static fn (SimpleNormalizer $normalizer, mixed $value) => $normalizer->normalizeMap($value),
+		];
+	}
+
+	/**
+	 * @dataProvider provideJsonVerifierEnabled
+	 */
+	public function testJsonVerifierEnabled (callable $call) : void
+	{
+		$verifier = $this->createMock(ValidJsonVerifier::class);
+
+		$verifier
+			->expects($this->once())
+			->method('ensureValidOnlyJsonTypes');
+
+		$normalizer = new SimpleNormalizer(
+			objectNormalizers: $this->createNormalizerObjectNormalizers("ok"),
+			isDebug: true,
+			validJsonVerifier: $verifier,
+		);
+
+		$call($normalizer, [
+			"nested" => [
+				"deeply" => [
+					"ohai" => new DummyVO(42),
+				],
+				"valid" => true,
+			],
+		]);
+	}
+
+
+	/**
+	 *
+	 */
+	public function testJsonVerifierDisabled () : void
+	{
+		$verifier = $this->createMock(ValidJsonVerifier::class);
+
+		$verifier
+			->expects($this->never())
+			->method('ensureValidOnlyJsonTypes');
+
+		$normalizer = new SimpleNormalizer(
+			objectNormalizers: $this->createNormalizerObjectNormalizers("ok"),
+			isDebug: false,
+			validJsonVerifier: $verifier,
+		);
+
+		$normalizer->normalize([
+			"key" => new DummyVO(42),
+		]);
+		self::assertTrue(true); // Just to ensure the test runs without exceptions
+	}
+
+
+	/**
+	 *
+	 */
+	public function testInvalidNormalizer () : void
+	{
+		$normalizer = new SimpleNormalizer(
+			objectNormalizers: $this->createNormalizerObjectNormalizers(new DummyVO(11)),
+			isDebug: true,
+			validJsonVerifier: new ValidJsonVerifier(),
+		);
+
+		$this->expectException(IncompleteNormalizationException::class);
+		$this->expectExceptionMessage("Found a JSON-incompatible value when normalizing. Found 'Tests\Torr\SimpleNormalizer\Fixture\DummyVO' at path '$', but expected only scalars, arrays and empty objects.");
+		$normalizer->normalize(new DummyVO(42));
+	}
+
+
+	/**
+	 *
+	 */
+	public function testInvalidNestedNormalizer () : void
+	{
+		$normalizer = new SimpleNormalizer(
+			objectNormalizers: $this->createNormalizerObjectNormalizers(new DummyVO(11)),
+			isDebug: true,
+			validJsonVerifier: new ValidJsonVerifier(),
+		);
+
+		$this->expectException(IncompleteNormalizationException::class);
+		$this->expectExceptionMessage("Found a JSON-incompatible value when normalizing. Found 'Tests\Torr\SimpleNormalizer\Fixture\DummyVO' at path '$.nested.deeply.ohai', but expected only scalars, arrays and empty objects.");
+		$normalizer->normalize([
+			"nested" => [
+				"deeply" => [
+					"ohai" => new DummyVO(42),
+				],
+				"valid" => true,
+			],
+		]);
+	}
+
+
+	/**
+	 *
+	 */
+	private function createNormalizerObjectNormalizers (mixed $returnValue) : ServiceLocator
+	{
+		return new ServiceLocator([
+			DummyVO::class => static fn () => new readonly class ($returnValue) implements SimpleObjectNormalizerInterface
+			{
+				public function __construct (
+					private mixed $returnValue,
+				) {}
+
+				public function normalize (object $value, array $context, SimpleNormalizer $normalizer) : mixed
+				{
+					return $this->returnValue;
+				}
+
+				public static function getNormalizedType () : string
+				{
+					return DummyVO::class;
+				}
+			},
+		]);
+	}
+}