diff --git a/src/XmlSitemapIndexWriter.php b/src/XmlSitemapIndexWriter.php
index 170a333..d288439 100644
--- a/src/XmlSitemapIndexWriter.php
+++ b/src/XmlSitemapIndexWriter.php
@@ -52,7 +52,7 @@ class XmlSitemapIndexWriter extends XmlSitemapWriter {
     for ($i = 1; $i <= $this->sitemap->chunks; $i++) {
       $url_options['query']['page'] = $i;
       $element = array(
-        'loc' => Url::fromRoute('xmlsitemap.sitemap_xml', [], $url_options),
+        'loc' => Url::fromRoute('xmlsitemap.sitemap_xml', [], $url_options)->toString(),
         // @todo Use the actual lastmod value of the chunk file.
         'lastmod' => gmdate($lastmod_format, REQUEST_TIME),
       );