diff --git a/core/lib/Drupal/Core/ParamConverter/ParamConverterManager.php b/core/lib/Drupal/Core/ParamConverter/ParamConverterManager.php
index 953acfb..c25cc87 100644
--- a/core/lib/Drupal/Core/ParamConverter/ParamConverterManager.php
+++ b/core/lib/Drupal/Core/ParamConverter/ParamConverterManager.php
@@ -79,8 +79,15 @@ public function applyToRequest($controller, Request $request) {
       }
 
       $param_name = $param->getName();
+      $param_value = $request->attributes->get($param_name);
 
-      $new_value = $this->convertTo($request->attributes->get($param_name), $param->getClass()->getName());
+      // No need to convert parameters that are already the objects they should
+      // be.
+      if (is_object($param_value) && $param->getClass()->isInstance($param_value)) {
+        continue;
+      }
+
+      $new_value = $this->convertTo($param_value, $param->getClass()->getName());
       $request->attributes->set($param_name, $new_value);
     }
   }