Hi all. I attempted to check my EPUB export with KindleViewer and got this error message:
W30009: Epub navigation file does not follow the specification. Please refer to Kindle Publishing Guidelines about usage of EPUB3 Navigation support. Tag should have value in file:
Then there’s a long file name. Yes, the value isn’t specified; it’s just a blank.
I go to the navigation guidelines on KDP: Navigation Guidelines
But this is incomprehensible to me. Could someone elucide what I should do in terms of Pressbooks?
Hi there. As a first step you could make sure that you receive emails when an export has validation errors. The email will then tell you where exactly there are issues and how to interpret them. How to fix the issue will depend on the nature of the issue. It usually involves looking in detail at the xhtml in your EPUB export file with the help of an EPUB editor like Calibre.
The issue that is likely being flagged by the KindleViewer here is that your book has a backmatter chapter that has no title (the very last backmatter chapter). You can either give this chapter a title, or omit it from the export as it does not have any content. A chapter without a title creates an issue in the table of contents, because there is no text to attach the link to.
Thanks! Indeed, there was an untitled backmatter chapter, and removing it made the KindleViewer accept the file. Nice catch. Thanks again.
In the meantime, I turned on validation error emails and immediately got two error reports. (A hanging div tag that isn’t visible in the html version of the chapter content and so probably exists in the xhtml wrapper.) Fixing these would certainly involve digging into the EPUB with Calibre.
However, even with these errors unfixed, KindleViewer didn’t flag them. Is there another stage in the process where these will pose a problem?
The errors in the validation log point to chapters that have double quotes in their title. These are not being escaped, which breaks the XHTML. KindleViewer seems to be forgiving towards this malformed XHTML, but other parsers may not. I filed a bug report and will ask the Tech team to look into this.
Any sense of the timeline for a fix?
FYI, I changed the double quotes to single ones and it eliminated the validation error on export.
And then, after tweaking some front matter, I get a new validation error. Can you tell me what this one is about?
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[time] => Fri Jun 14:07:36 2026
[user] => michaelerard
[site_url] => https://pressbooks.pub/lbme
[blog_id] => 26963
[theme] => Clarke
[formatted_validation_report] => EPUB VALIDATION REPORT
It was front matter without a title. Fixed it