diff --git a/core/modules/update/src/UpdateProcessor.php b/core/modules/update/src/UpdateProcessor.php
index d37f643..f129845 100644
--- a/core/modules/update/src/UpdateProcessor.php
+++ b/core/modules/update/src/UpdateProcessor.php
@@ -161,7 +161,7 @@ public function processFetchTask($project) {
     $project_name = $project['name'];
 
     if (empty($this->failed[$fetch_url_base]) || $this->failed[$fetch_url_base] < $max_fetch_attempts) {
-      $data = $this->updateFetcher->fetchProjectData($project, $site_key);
+      $data = (string) $this->updateFetcher->fetchProjectData($project, $site_key);
     }
     if (!empty($data)) {
       $available = $this->parseXml($data);