diff --git a/core/modules/serialization/lib/Drupal/serialization/RegisterSerializationClassesCompilerPass.php b/core/modules/serialization/lib/Drupal/serialization/RegisterSerializationClassesCompilerPass.php
index 60c2424..fb87bab 100644
--- a/core/modules/serialization/lib/Drupal/serialization/RegisterSerializationClassesCompilerPass.php
+++ b/core/modules/serialization/lib/Drupal/serialization/RegisterSerializationClassesCompilerPass.php
@@ -45,7 +45,7 @@ public function process(ContainerBuilder $container) {
 
     // Find all serialization formats known.
     $formats = array();
-    foreach ($container->findTaggedServiceIds('encoder') as $id => $attributes) {
+    foreach ($container->findTaggedServiceIds('encoder') as $attributes) {
       $formats[] = $attributes[0]['format'];
     }
     $container->setParameter('serializer.formats', $formats);