diff --git a/core/lib/Drupal/Core/Routing/Router.php b/core/lib/Drupal/Core/Routing/Router.php
index 29b2ee10dd..ef7bea019d 100644
--- a/core/lib/Drupal/Core/Routing/Router.php
+++ b/core/lib/Drupal/Core/Routing/Router.php
@@ -4,6 +4,7 @@
 
 use Drupal\Core\Path\CurrentPathStack;
 use Symfony\Cmf\Component\Routing\Enhancer\RouteEnhancerInterface;
+use \Drupal\Core\Routing\Enhancer\RouteEnhancerInterface as LegacyRouteEnhancerInterface;
 use Symfony\Cmf\Component\Routing\LazyRouteCollection;
 use Symfony\Cmf\Component\Routing\RouteObjectInterface;
 use Symfony\Cmf\Component\Routing\RouteProviderInterface as BaseRouteProviderInterface;
@@ -254,9 +255,10 @@ protected function getInitialRouteCollection(Request $request) {
    */
   protected function applyRouteEnhancers($defaults, Request $request) {
     foreach ($this->enhancers as $enhancer) {
-      if ($enhancer instanceof RouteEnhancerInterface || $enhancer->applies($defaults[RouteObjectInterface::ROUTE_OBJECT])) {
-        $defaults = $enhancer->enhance($defaults, $request);
+      if ($enhancer instanceof LegacyRouteEnhancerInterface && !$enhancer->applies($defaults[RouteObjectInterface::ROUTE_OBJECT)) {
+        continue;
       }
+      $defaults = $enhancer->enhance($defaults, $request);
     }
 
     return $defaults;
diff --git a/core/tests/Drupal/KernelTests/Core/Routing/ContentNegotiationRoutingTest.php b/core/tests/Drupal/KernelTests/Core/Routing/ContentNegotiationRoutingTest.php
index 87d924654e..052e74f35b 100644
--- a/core/tests/Drupal/KernelTests/Core/Routing/ContentNegotiationRoutingTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Routing/ContentNegotiationRoutingTest.php
@@ -49,36 +49,36 @@ public function testContentRouting() {
       // ['path', 'accept', 'content-type'],
 
       // Extension is part of the route path. Constant Content-type.
-//      ['conneg/simple.json', '', 'application/json'],
-//      ['conneg/simple.json', 'application/xml', 'application/json'],
-//      ['conneg/simple.json', 'application/json', 'application/json'],
+      ['conneg/simple.json', '', 'application/json'],
+      ['conneg/simple.json', 'application/xml', 'application/json'],
+      ['conneg/simple.json', 'application/json', 'application/json'],
       // No extension. Constant Content-type.
-//      ['conneg/html', '', 'text/html'],
-//      ['conneg/html', '*/*', 'text/html'],
-//      ['conneg/html', 'application/xml', 'text/html'],
-//      ['conneg/html', 'text/xml', 'text/html'],
-//      ['conneg/html', 'text/html', 'text/html'],
+      ['conneg/html', '', 'text/html'],
+      ['conneg/html', '*/*', 'text/html'],
+      ['conneg/html', 'application/xml', 'text/html'],
+      ['conneg/html', 'text/xml', 'text/html'],
+      ['conneg/html', 'text/html', 'text/html'],
       // Dynamic extension. Linked Content-type.
       ['conneg/html?_format=json', '', 'application/json'],
-//      ['conneg/html?_format=json', '*/*', 'application/json'],
-//      ['conneg/html?_format=json', 'application/xml', 'application/json'],
-//      ['conneg/html?_format=json', 'application/json', 'application/json'],
-//      ['conneg/html?_format=xml', '', 'application/xml'],
-//      ['conneg/html?_format=xml', '*/*', 'application/xml'],
-//      ['conneg/html?_format=xml', 'application/json', 'application/xml'],
-//      ['conneg/html?_format=xml', 'application/xml', 'application/xml'],
+      ['conneg/html?_format=json', '*/*', 'application/json'],
+      ['conneg/html?_format=json', 'application/xml', 'application/json'],
+      ['conneg/html?_format=json', 'application/json', 'application/json'],
+      ['conneg/html?_format=xml', '', 'application/xml'],
+      ['conneg/html?_format=xml', '*/*', 'application/xml'],
+      ['conneg/html?_format=xml', 'application/json', 'application/xml'],
+      ['conneg/html?_format=xml', 'application/xml', 'application/xml'],
 
       // Path with a variable. Variable contains a period.
-//      ['conneg/plugin/plugin.id', '', 'text/html'],
-//      ['conneg/plugin/plugin.id', '*/*', 'text/html'],
-//      ['conneg/plugin/plugin.id', 'text/xml', 'text/html'],
-//      ['conneg/plugin/plugin.id', 'text/html', 'text/html'],
+      ['conneg/plugin/plugin.id', '', 'text/html'],
+      ['conneg/plugin/plugin.id', '*/*', 'text/html'],
+      ['conneg/plugin/plugin.id', 'text/xml', 'text/html'],
+      ['conneg/plugin/plugin.id', 'text/html', 'text/html'],
 
       // Alias with extension pointing to no extension/constant content-type.
-//      ['alias.html', '', 'text/html'],
-//      ['alias.html', '*/*', 'text/html'],
-//      ['alias.html', 'text/xml', 'text/html'],
-//      ['alias.html', 'text/html', 'text/html'],
+      ['alias.html', '', 'text/html'],
+      ['alias.html', '*/*', 'text/html'],
+      ['alias.html', 'text/xml', 'text/html'],
+      ['alias.html', 'text/html', 'text/html'],
     ];
 
     foreach ($tests as $test) {