diff --git a/bundle/Controller/SitemapController.php b/bundle/Controller/SitemapController.php
index a27581e..b50a0f9 100644
--- a/bundle/Controller/SitemapController.php
+++ b/bundle/Controller/SitemapController.php
@@ -141,7 +141,7 @@ protected function fillSitemap(DOMDocument $sitemap, DOMElement $root, SearchRes
                 continue;
             }
 
-            $modified = $location->contentInfo->modificationDate->format('c');
+            $modified = $location->contentInfo->modificationDate?->format('c');
             $loc = $sitemap->createElement('loc', $url);
             $lastmod = $sitemap->createElement('lastmod', $modified);
             $urlElt = $sitemap->createElement('url');