diff --git a/src/Symfony/Component/Config/Definition/Dumper/XmlReferenceDumper.php b/src/Symfony/Component/Config/Definition/Dumper/XmlReferenceDumper.php
index ff8c35388870..da0a971213ee 100644
--- a/src/Symfony/Component/Config/Definition/Dumper/XmlReferenceDumper.php
+++ b/src/Symfony/Component/Config/Definition/Dumper/XmlReferenceDumper.php
@@ -91,7 +91,7 @@ private function writeNode(NodeInterface $node, int $depth = 0, bool $root = fal
                 }
 
                 if ($prototype instanceof PrototypedArrayNode) {
-                    $prototype->setName($key);
+                    $prototype->setName($key ?? '');
                     $children = [$key => $prototype];
                 } elseif ($prototype instanceof ArrayNode) {
                     $children = $prototype->getChildren();