diff --git a/src/Symfony/Component/DomCrawler/Crawler.php b/src/Symfony/Component/DomCrawler/Crawler.php
index e13b94fb5838b..0a26cd2641356 100644
--- a/src/Symfony/Component/DomCrawler/Crawler.php
+++ b/src/Symfony/Component/DomCrawler/Crawler.php
@@ -1110,7 +1110,7 @@ private function createDOMXPath(\DOMDocument $document, array $prefixes = array(
     /**
      * @throws \InvalidArgumentException
      */
-    private function discoverNamespace(\DOMXPath $domxpath, string $prefix): string
+    private function discoverNamespace(\DOMXPath $domxpath, string $prefix): ?string
     {
         if (isset($this->namespaces[$prefix])) {
             return $this->namespaces[$prefix];
@@ -1122,6 +1122,8 @@ private function discoverNamespace(\DOMXPath $domxpath, string $prefix): string
         if ($node = $namespaces->item(0)) {
             return $node->nodeValue;
         }
+
+        return null;
     }
 
     private function findNamespacePrefixes(string $xpath): array