django Sitemap class alternates not rendering sitemap.xml

Question

I have a problem with adding alternates attribute to my sitemaps.py

sitemaps.py

class StaticViewSitemap(sitemaps.Sitemap):
    priority = 0.5
    changefreq = 'weekly' 
    protocol = 'https' 
    i18n = True 
    alternates = True

def alternate_hreflangs(self, obj):
    return [('en-us', obj.alternative_object_url), ]

def items(self):
    return ['index','about', 'contact', 'why', 'product', 'tender-management', 'e-sourcing', 'network', 'faq']

def location(self, item):
    return reverse(item)

when I set the alternates = True my sitemap.xml file broke and show a string file

https://example.com/en/weekly0.5https://example.com/en/about/weekly0.5https://example.com/en/contact/weekly0.5

Instead of

<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
<url>
<loc>https://example.com/en/</loc>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>https://example.com/en/about/</loc>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>https://example.com/en/contact/</loc>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>

I get this problem only when adding the alternates = True my sitemap.xml