diff --git a/.idea/problem-details-symfony-bundle.iml b/.idea/problem-details-symfony-bundle.iml
index 2446eea..43a3eb2 100644
--- a/.idea/problem-details-symfony-bundle.iml
+++ b/.idea/problem-details-symfony-bundle.iml
@@ -3,6 +3,7 @@
   <component name="NewModuleRootManager">
     <content url="file://$MODULE_DIR$">
       <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" packagePrefix="Phauthentic\Symfony\ProblemDetails\" />
+      <sourceFolder url="file://$MODULE_DIR$/tests" isTestSource="true" />
       <sourceFolder url="file://$MODULE_DIR$/tests" isTestSource="true" packagePrefix="Phauthentic\Symfony\ProblemDetails\Tests\" />
       <excludeFolder url="file://$MODULE_DIR$/vendor/colinodell/json5" />
       <excludeFolder url="file://$MODULE_DIR$/vendor/composer" />
diff --git a/phpstan.neon b/phpstan.neon
index 3898cf4..7dbf278 100644
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -4,3 +4,4 @@ parameters:
         - src
     parallel:
         maximumNumberOfProcesses: 4
+    treatPhpDocTypesAsCertain: false
diff --git a/src/ProblemDetailsResponse.php b/src/ProblemDetailsResponse.php
index 9f2869b..8a203d6 100644
--- a/src/ProblemDetailsResponse.php
+++ b/src/ProblemDetailsResponse.php
@@ -53,7 +53,7 @@ public static function create(
         ];
 
         if (!empty($detail)) {
-            $data['detail'] = $instance;
+            $data['detail'] = $detail;
         }
 
         if (!empty($instance)) {
@@ -87,12 +87,8 @@ public static function assertReservedResponseFields(array $extensions): void
     /**
      * Validates if the given status code is a valid client-side (4xx) or server-side (5xx) error.
      */
-    protected static function assertValidStatusCode(?int $statusCode): void
+    protected static function assertValidStatusCode(int $statusCode): void
     {
-        if (!$statusCode) {
-            return;
-        }
-
         if (!($statusCode >= 400 && $statusCode < 500) && !($statusCode >= 500 && $statusCode < 600)) {
             throw new LogicException(sprintf(
                 'Invalid status code %s provided for a Problem Details response. '
diff --git a/src/ThrowableToProblemDetailsKernelListener.php b/src/ThrowableToProblemDetailsKernelListener.php
index 12cc7a2..64b8bb9 100644
--- a/src/ThrowableToProblemDetailsKernelListener.php
+++ b/src/ThrowableToProblemDetailsKernelListener.php
@@ -32,7 +32,7 @@ public function __construct(
         protected array $exceptionConverters = []
     ) {
         if (empty($this->exceptionConverters)) {
-            throw new InvalidArgumentException('No exception converter passed!');
+            throw new InvalidArgumentException('At least one converter must be provided');
         }
     }
 
@@ -49,6 +49,10 @@ private function processConverters(ExceptionEvent $event): void
     {
         $throwable = $event->getThrowable();
         foreach ($this->exceptionConverters as $exceptionConverter) {
+            if (!$exceptionConverter instanceof ExceptionConverterInterface) {
+                throw new InvalidArgumentException('All converters must implement ' . ExceptionConverterInterface::class);
+            }
+
             if (!$exceptionConverter->canHandle($throwable)) {
                 continue;
             }
diff --git a/tests/Unit/ExceptionConversion/ValidationFailedExceptionConverterTest.php b/tests/Unit/ExceptionConversion/ValidationFailedExceptionConverterTest.php
index 7219ad4..70ac422 100644
--- a/tests/Unit/ExceptionConversion/ValidationFailedExceptionConverterTest.php
+++ b/tests/Unit/ExceptionConversion/ValidationFailedExceptionConverterTest.php
@@ -11,8 +11,11 @@
 use Phauthentic\Symfony\ProblemDetails\Validation\ValidationErrorsBuilder;
 use PHPUnit\Framework\Attributes\Test;
 use PHPUnit\Framework\TestCase;
+use ReflectionMethod;
+use RuntimeException;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpKernel\Event\ExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\UnprocessableEntityHttpException;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
 use Symfony\Component\Validator\ConstraintViolation;
 use Symfony\Component\Validator\ConstraintViolationList;
@@ -102,4 +105,22 @@ private function getConstraintViolationList(): ConstraintViolationList
 
         return new ConstraintViolationList([$violation1, $violation2]);
     }
+
+    #[Test]
+    public function testExtractValidationFailedExceptionThrowsRuntimeException(): void
+    {
+        // Arrange
+        $exception = new UnprocessableEntityHttpException('Validation failed', new Exception(), 0, []);
+        $kernel = $this->createMock(HttpKernelInterface::class);
+        $request = new Request([], [], [], [], [], ['REQUEST_URI' => '/profile/1']);
+        $request->headers->add(['Accept' => 'application/json']);
+        $event = new ExceptionEvent($kernel, $request, HttpKernelInterface::MAIN_REQUEST, $exception);
+
+        // Assert
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('ValidationFailedException not found');
+
+        // Act
+        $this->converter->convertExceptionToErrorDetails($exception, $event);
+    }
 }
diff --git a/tests/Unit/ProblemDetailsResponseTest.php b/tests/Unit/ProblemDetailsResponseTest.php
new file mode 100644
index 0000000..1e11b20
--- /dev/null
+++ b/tests/Unit/ProblemDetailsResponseTest.php
@@ -0,0 +1,117 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Phauthentic\Symfony\ProblemDetails\Tests\Unit;
+
+use InvalidArgumentException;
+use LogicException;
+use PHPUnit\Framework\TestCase;
+use Phauthentic\Symfony\ProblemDetails\ProblemDetailsResponse;
+
+class ProblemDetailsResponseTest extends TestCase
+{
+    public function testCreateValidResponse(): void
+    {
+        // Arrange
+        $status = 422;
+        $type = 'https://example.com/probs/out-of-credit';
+        $title = 'You do not have enough credit.';
+        $detail = 'Your current balance is 30, but that costs 50.';
+        $instance = '/account/12345/msgs/abc';
+        $extensions = ['balance' => 30, 'accounts' => ['/account/12345', '/account/67890']];
+
+        // Act
+        $response = ProblemDetailsResponse::create(
+            status: $status,
+            type: $type,
+            title: $title,
+            detail: $detail,
+            instance: $instance,
+            extensions: $extensions
+        );
+
+        // Assert
+        $this->assertEquals($status, $response->getStatusCode());
+        $this->assertEquals('application/problem+json', $response->headers->get('Content-Type'));
+        $this->assertJsonStringEqualsJsonString(
+            json_encode([
+                'status' => $status,
+                'type' => $type,
+                'title' => $title,
+                'detail' => $detail,
+                'instance' => $instance,
+                'balance' => 30,
+                'accounts' => ['/account/12345', '/account/67890']
+            ], JSON_THROW_ON_ERROR),
+            $response->getContent()
+        );
+    }
+
+    public function testCreateResponseWithReservedFieldInExtensions(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('The key "status" is a reserved key and cannot be used as an extension.');
+
+        // Arrange
+        $status = 422;
+        $type = 'https://example.com/probs/out-of-credit';
+        $title = 'You do not have enough credit.';
+        $detail = 'Your current balance is 30, but that costs 50.';
+        $instance = '/account/12345/msgs/abc';
+        $extensions = ['status' => 'reserved'];
+
+        // Act
+        ProblemDetailsResponse::create(
+            status: $status,
+            type: $type,
+            title: $title,
+            detail: $detail,
+            instance: $instance,
+            extensions: $extensions
+        );
+    }
+
+    public function testCreateResponseWithInvalidStatusCode(): void
+    {
+        $this->expectException(LogicException::class);
+        $this->expectExceptionMessage('Invalid status code 200 provided for a Problem Details response.');
+
+        // Arrange
+        $status = 200;
+        $type = 'https://example.com/probs/out-of-credit';
+        $title = 'You do not have enough credit.';
+        $detail = 'Your current balance is 30, but that costs 50.';
+        $instance = '/account/12345/msgs/abc';
+
+        // Act
+        ProblemDetailsResponse::create(
+            status: $status,
+            type: $type,
+            title: $title,
+            detail: $detail,
+            instance: $instance
+        );
+    }
+
+    public function testCreateResponseWithMinimalParameters(): void
+    {
+        // Arrange
+        $status = 500;
+
+        // Act
+        $response = ProblemDetailsResponse::create($status);
+
+        // Assert
+        $this->assertEquals($status, $response->getStatusCode());
+        $this->assertEquals('application/problem+json', $response->headers->get('Content-Type'));
+        $this->assertJsonStringEqualsJsonString(
+            json_encode([
+                'status' => $status,
+                'type' => 'about:blank',
+                'title' => null,
+            ], JSON_THROW_ON_ERROR),
+            $response->getContent()
+        );
+    }
+}
diff --git a/tests/Unit/ThrowableToProblemDetailsKernelListenerTest.php b/tests/Unit/ThrowableToProblemDetailsKernelListenerTest.php
index 6a0f490..2b978c0 100644
--- a/tests/Unit/ThrowableToProblemDetailsKernelListenerTest.php
+++ b/tests/Unit/ThrowableToProblemDetailsKernelListenerTest.php
@@ -5,6 +5,7 @@
 namespace Phauthentic\Symfony\ProblemDetails\Tests\Unit;
 
 use Exception;
+use InvalidArgumentException;
 use Phauthentic\Symfony\ProblemDetails\ExceptionConversion\GenericThrowableConverter;
 use Phauthentic\Symfony\ProblemDetails\ProblemDetailsFactory;
 use PHPUnit\Framework\Attributes\DataProvider;
@@ -65,4 +66,33 @@ public function testOnKernelException(string $environment, bool $shouldHaveTrace
             $this->assertArrayNotHasKey('trace', $data);
         }
     }
+
+    #[Test]
+    public function testInstantiationWithoutConverters(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('At least one converter must be provided');
+
+        new ThrowableToProblemDetailsKernelListener([]);
+    }
+
+    #[Test]
+    public function testInstantiationWithoutValidConverter(): void
+    {
+        // Arrange
+        $throwable = new Exception('Unmapped exception');
+        $kernel = $this->createMock(HttpKernelInterface::class);
+        $request = new Request(
+            server: ['HTTP_ACCEPT' => 'application/json']
+        );
+        $event = new ExceptionEvent($kernel, $request, HttpKernelInterface::MAIN_REQUEST, $throwable);
+
+        // Expect
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('All converters must implement Phauthentic\Symfony\ProblemDetails\ExceptionConversion\ExceptionConverterInterface');
+
+        // Act
+        $listener = new ThrowableToProblemDetailsKernelListener([new \stdClass()]);
+        $listener->onKernelException($event);
+    }
 }