Quick answer
A standard sitemap uses the namespace http://www.sitemaps.org/schemas/sitemap/0.9 on its <urlset> or <sitemapindex> root. NOTABIS reports sitemap-invalid-namespace when a declared sitemap supplies a different non-empty default namespace. Correct the namespace or the sitemap generator before relying on the file for discovery.
What the issue means
The namespace tells an XML parser which vocabulary the elements belong to:
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url><loc>https://example.com/</loc></url>
</urlset>
An absent default namespace is handled differently by the current NOTABIS parser from a present, incorrect one. Extensions such as image or video namespaces can appear in addition to the standard sitemap namespace; they do not replace it.
Why it matters for SEO
A wrong namespace can make a sitemap parser treat standard elements as an unknown vocabulary, reducing the chance that the file is processed as intended. It is a format error, not a direct ranking penalty, and it does not tell you whether the listed pages themselves are valid or indexable.
How NOTABIS detects it
NOTABIS parses each fetched sitemap with SimpleXML and reads its default namespace. When the value is non-empty and is not http://www.sitemaps.org/schemas/sitemap/0.9, it records the URL and creates sitemap-invalid-namespace for a sitemap declared in robots.txt. The check is sitemap-scoped; conventional fallback paths can be recorded in diagnostics even when they are not declared. Malformed XML and invalid roots are handled separately.
How to check it yourself
Open the raw sitemap and inspect the root element’s xmlns value. Validate a sitemap index and each child sitemap, not only the index URL. Check that redirects, content type, and compression do not change the body delivered publicly.
How to fix it
Correct the generator or template so every sitemap root uses the standard namespace. Keep extension namespaces as additional declarations when needed:
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
xmlns:image="http://www.google.com/schemas/sitemap-image/1.1">
Regenerate the file, fetch the public URL, and validate the XML before updating robots.txt or Search Console submissions. Remove stale declared locations that no longer represent a sitemap.
When this is intentional or does not need fixing
An XML document that is not a sitemap can use another namespace, but do not declare it as one. Extension data may use its own namespace while retaining the standard sitemap namespace on the root. A sitemap with no default namespace may still be parsed by some consumers, but aligning it with the standard is safer and clearer.
Related issues
Read sitemap access, invalid sitemap content types, and sitemap unavailability. The sitemaps.org protocol defines the standard namespace.
Run a NOTABIS audit
NOTABIS records declared sitemap namespace values and identifies files that do not use the standard sitemap vocabulary.