diff --git a/src/Symfony/Component/Security/Core/Authorization/TraceableAccessDecisionManager.php b/src/Symfony/Component/Security/Core/Authorization/TraceableAccessDecisionManager.php
index 7a9c4d0cfb..3b5004edf2 100644
--- a/src/Symfony/Component/Security/Core/Authorization/TraceableAccessDecisionManager.php
+++ b/src/Symfony/Component/Security/Core/Authorization/TraceableAccessDecisionManager.php
@@ -107,4 +107,6 @@ class TraceableAccessDecisionManager implements AccessDecisionManagerInterface
     }
 }
 
-class_alias(TraceableAccessDecisionManager::class, DebugAccessDecisionManager::class);
+if (!class_exists(DebugAccessDecisionManager::class, false)) {
+    class_alias(TraceableAccessDecisionManager::class, DebugAccessDecisionManager::class);
+}