Canonical is malformed or empty
This means that the URL in question has a canonical element specified, but the canonical URL is missing or invalid.
Why is this important?
If search engines encounter conflicting canonical URLs, they will ignore the canonical instruction entirely. This could result in the indexing of pages which you do not wish to be indexed, such as duplicate content.
If a canonical URL has not been defined correctly, search engines will not be able to deduce what it 'should' be, and therefore the canonical instruction will be completely ignored.
What does the Hint check?
This Hint will trigger for any internal URL that specifies a canonical URL which is invalid or undefined.
Examples that trigger this Hint:
Consider the URL: https://example.com/page-a
The Hint would trigger for this URL if it the canonical URL is undefined;
Undefined canonical tag in the <head>:
OR undefined canonical in the HTTP header
The Hint would also trigger for this URL if it the canonical URL is invalid;
Invalid canonical tag in the <head>:
OR invalid canonical in the HTTP header
How do you resolve this issue?
If this Hint triggers, it indicates a configuration issue on the URL in question. It may be as straightforward as logging into the CMS and fixing the canonical, or it may require some developer assistance. If it has happened across lots of pages at once, then it may be a CMS issue that can be quickly fixed by adjusting some page templates.