diff --git a/module/Core/src/Action/QrCodeAction.php b/module/Core/src/Action/QrCodeAction.php
index 5d0549eb..3970d740 100644
--- a/module/Core/src/Action/QrCodeAction.php
+++ b/module/Core/src/Action/QrCodeAction.php
@@ -3,6 +3,8 @@ namespace Shlinkio\Shlink\Core\Action;
 
 use Acelaya\ZsmAnnotatedServices\Annotation\Inject;
 use Endroid\QrCode\QrCode;
+use Interop\Http\ServerMiddleware\DelegateInterface;
+use Interop\Http\ServerMiddleware\MiddlewareInterface;
 use Psr\Http\Message\ResponseInterface as Response;
 use Psr\Http\Message\ServerRequestInterface as Request;
 use Psr\Log\LoggerInterface;
@@ -12,7 +14,6 @@ use Shlinkio\Shlink\Core\Exception\InvalidShortCodeException;
 use Shlinkio\Shlink\Core\Service\UrlShortener;
 use Shlinkio\Shlink\Core\Service\UrlShortenerInterface;
 use Zend\Expressive\Router\RouterInterface;
-use Zend\Stratigility\MiddlewareInterface;
 
 class QrCodeAction implements MiddlewareInterface
 {
@@ -48,42 +49,26 @@ class QrCodeAction implements MiddlewareInterface
     }
 
     /**
-     * Process an incoming request and/or response.
-     *
-     * Accepts a server-side request and a response instance, and does
-     * something with them.
-     *
-     * If the response is not complete and/or further processing would not
-     * interfere with the work done in the middleware, or if the middleware
-     * wants to delegate to another process, it can use the `$out` callable
-     * if present.
-     *
-     * If the middleware does not return a value, execution of the current
-     * request is considered complete, and the response instance provided will
-     * be considered the response to return.
-     *
-     * Alternately, the middleware may return a response instance.
-     *
-     * Often, middleware will `return $out();`, with the assumption that a
-     * later middleware will return a response.
+     * Process an incoming server request and return a response, optionally delegating
+     * to the next middleware component to create the response.
      *
      * @param Request $request
-     * @param Response $response
-     * @param null|callable $out
-     * @return null|Response
+     * @param DelegateInterface $delegate
+     *
+     * @return Response
      */
-    public function __invoke(Request $request, Response $response, callable $out = null)
+    public function process(Request $request, DelegateInterface $delegate)
     {
         // Make sure the short URL exists for this short code
         $shortCode = $request->getAttribute('shortCode');
         try {
             $shortUrl = $this->urlShortener->shortCodeToUrl($shortCode);
-            if (! isset($shortUrl)) {
-                return $out($request, $response->withStatus(404), 'Not Found');
+            if ($shortUrl === null) {
+                return $delegate->process($request);
             }
         } catch (InvalidShortCodeException $e) {
             $this->logger->warning('Tried to create a QR code with an invalid short code' . PHP_EOL . $e);
-            return $out($request, $response->withStatus(404), 'Not Found');
+            return $delegate->process($request);
         }
 
         $path = $this->router->generateUri('long-url-redirect', ['shortCode' => $shortCode]);
@@ -101,13 +86,11 @@ class QrCodeAction implements MiddlewareInterface
      */
     protected function getSizeParam(Request $request)
     {
-        $size = intval($request->getAttribute('size', 300));
+        $size = (int) $request->getAttribute('size', 300);
         if ($size < 50) {
             return 50;
-        } elseif ($size > 1000) {
-            return 1000;
         }
 
-        return $size;
+        return $size > 1000 ? 1000 : $size;
     }
 }
diff --git a/module/Core/test/Action/QrCodeActionTest.php b/module/Core/test/Action/QrCodeActionTest.php
index 6ef8a5f5..dc005b11 100644
--- a/module/Core/test/Action/QrCodeActionTest.php
+++ b/module/Core/test/Action/QrCodeActionTest.php
@@ -9,7 +9,7 @@ use Shlinkio\Shlink\Core\Action\QrCodeAction;
 use Shlinkio\Shlink\Core\Entity\ShortUrl;
 use Shlinkio\Shlink\Core\Exception\InvalidShortCodeException;
 use Shlinkio\Shlink\Core\Service\UrlShortener;
-use Zend\Diactoros\Response;
+use ShlinkioTest\Shlink\Common\Util\TestUtils;
 use Zend\Diactoros\ServerRequestFactory;
 use Zend\Expressive\Router\RouterInterface;
 
@@ -37,19 +37,18 @@ class QrCodeActionTest extends TestCase
     /**
      * @test
      */
-    public function aNonexistentShortCodeWillReturnNotFoundResponse()
+    public function aNotFoundShortCodeWillDelegateIntoNextMiddleware()
     {
         $shortCode = 'abc123';
         $this->urlShortener->shortCodeToUrl($shortCode)->willReturn(null)->shouldBeCalledTimes(1);
+        $delegate = TestUtils::createDelegateMock();
 
-        $resp = $this->action->__invoke(
+        $this->action->process(
             ServerRequestFactory::fromGlobals()->withAttribute('shortCode', $shortCode),
-            new Response(),
-            function ($req, $resp) {
-                return $resp;
-            }
+            $delegate->reveal()
         );
-        $this->assertEquals(404, $resp->getStatusCode());
+
+        $delegate->process(Argument::any())->shouldHaveBeenCalledTimes(1);
     }
 
     /**
@@ -60,15 +59,14 @@ class QrCodeActionTest extends TestCase
         $shortCode = 'abc123';
         $this->urlShortener->shortCodeToUrl($shortCode)->willThrow(InvalidShortCodeException::class)
                                                        ->shouldBeCalledTimes(1);
+        $delegate = TestUtils::createDelegateMock();
 
-        $resp = $this->action->__invoke(
+        $this->action->process(
             ServerRequestFactory::fromGlobals()->withAttribute('shortCode', $shortCode),
-            new Response(),
-            function ($req, $resp) {
-                return $resp;
-            }
+            $delegate->reveal()
         );
-        $this->assertEquals(404, $resp->getStatusCode());
+
+        $delegate->process(Argument::any())->shouldHaveBeenCalledTimes(1);
     }
 
     /**
@@ -78,16 +76,15 @@ class QrCodeActionTest extends TestCase
     {
         $shortCode = 'abc123';
         $this->urlShortener->shortCodeToUrl($shortCode)->willReturn(new ShortUrl())->shouldBeCalledTimes(1);
+        $delegate = TestUtils::createDelegateMock();
 
-        $resp = $this->action->__invoke(
+        $resp = $this->action->process(
             ServerRequestFactory::fromGlobals()->withAttribute('shortCode', $shortCode),
-            new Response(),
-            function ($req, $resp) {
-                return $resp;
-            }
+            $delegate->reveal()
         );
 
         $this->assertInstanceOf(QrCodeResponse::class, $resp);
         $this->assertEquals(200, $resp->getStatusCode());
+        $delegate->process(Argument::any())->shouldHaveBeenCalledTimes(0);
     }
 }