diff --git a/src/Symfony/Component/ErrorRenderer/Exception/FlattenException.php b/src/Symfony/Component/ErrorRenderer/Exception/FlattenException.php
index 9ec583f7b7..154a1450a1 100644
--- a/src/Symfony/Component/ErrorRenderer/Exception/FlattenException.php
+++ b/src/Symfony/Component/ErrorRenderer/Exception/FlattenException.php
@@ -374,3 +374,18 @@ class FlattenException
         return rtrim($message);
     }
 }
+
+namespace Symfony\Component\Debug\Exception;
+
+if (!class_exists(FlattenException::class, false)) {
+    class_alias(\Symfony\Component\ErrorRenderer\Exception\FlattenException::class, FlattenException::class);
+}
+
+if (false) {
+    /**
+     * @deprecated since Symfony 4.4, use Symfony\Component\ErrorRenderer\Exception\FlattenException instead.
+     */
+    class FlattenException extends \Symfony\Component\ErrorRenderer\Exception\FlattenException
+    {
+    }
+}