Common EPUB Errors and How to Fix Them

Few things are more frustrating than finishing your ebook, uploading it, and getting an error message — or worse, a rejection. EPUB errors are one of the most common roadblocks self-published authors hit, and the error messages are often technical and unhelpful. The good news is that most EPUB problems fall into a handful of familiar categories, and once you know what they are, they're far less intimidating. Here are the most common EPUB errors in 2026, what each one actually means, and how to fix it before it costs you a launch day.

1. A broken or missing table of contents

This is the single most common EPUB complaint. Your ebook uploads, but readers can't jump between chapters, or the retailer flags a missing navigation document. In EPUB, the table of contents isn't just a page — it's a special navigation file that tells e-readers how the book is structured.

How to fix it: the table of contents should be generated from your actual heading styles, not typed by hand. If you used proper "Heading 1" styles for your chapters when writing, a good formatting process builds the navigation automatically. If your TOC is broken, the cleanest fix is usually to re-generate it from correctly styled headings rather than patching the file.

2. A cover that isn't properly declared

Your cover image might be sitting inside the EPUB, but if it isn't declared as the cover in the book's metadata, retailers may not recognise it — leading to a "missing cover" warning or a blank thumbnail. There's a difference between an image that's simply in the file and one that's correctly registered as the official cover.

How to fix it: the cover needs to be referenced correctly in the EPUB's package file so retailers know it's the cover. This is a small technical detail that's easy to miss and easy to correct once you know to look for it.

3. Invalid or missing metadata

Metadata is the information about your book: title, author, language, and a unique identifier. Retailers require certain fields, and if they're missing or malformed, your upload can be rejected. A frequent culprit is a missing language declaration or an absent unique identifier.

How to fix it: make sure your title, author name, language, and a unique identifier (an ISBN or a generated ID) are all present and correctly formatted in the metadata. These are quick to add but easy to overlook.

4. Files that fail EPUB validation

EPUBs follow a strict standard, and validation tools check your file against it. Errors here often have cryptic codes and point to problems in the underlying markup — malformed HTML, broken internal links, unreferenced files, or a mimetype that isn't set up correctly. These are the errors that most often cause an outright rejection.

How to fix it: run your file through a validator to see exactly what's flagged, then address each item. Structural errors are the hardest to fix by hand — if there are many, re-formatting the book properly is usually faster and safer than repairing a broken file piece by piece.

5. Images that are too large or wrongly sized

Oversized images bloat your file, slow down e-readers, and can push you into higher delivery fees on some platforms. Images that are too small look blurry; images in the wrong format can fail to display. Cover images in particular have recommended dimensions that retailers expect.

How to fix it: size images appropriately for screen reading, use the right formats, and make sure your cover meets the retailer's recommended dimensions. Balancing quality against file size is part of good ebook formatting.

6. Fonts that don't embed or display correctly

If you used a special font and it isn't embedded properly, readers won't see it — the book falls back to a default, and your careful typography is lost. Worse, some font issues trigger validation errors. Embedded fonts also need to be licensed for embedding.

How to fix it: only embed fonts you're licensed to embed, embed them correctly, and remember that for most reflowable ebooks, letting the reader's device choose the font is often the more reliable choice.

7. Broken internal links and navigation

Links within your book — to endnotes, other chapters, or a contents page — can break during conversion, sending readers to the wrong place or nowhere at all. This is especially common in books converted quickly from Word.

How to fix it: test every internal link after formatting. Broken links usually trace back to a messy conversion, so a clean formatting process prevents most of them.

The simplest way to avoid all of these

Here's the honest truth: nearly every error on this list comes from one of two things — a rushed conversion from Word, or small technical details missed during formatting. You can catch them by always validating your EPUB before you upload, and you can prevent most of them by formatting your book properly in the first place. If your file is throwing multiple errors and you'd rather not spend days debugging cryptic codes, a professional EPUB conversion service will deliver a clean, validated file that passes on the first upload — so your launch day goes smoothly instead of stalling on an error message.

The bottom line

EPUB errors feel scary because the messages are technical, but they almost always come down to a handful of fixable issues: the table of contents, the cover, metadata, validation, images, fonts, and links. Check your file before you publish, fix what's flagged, and if the list is long, don't be afraid to let a professional handle it. A clean EPUB is the difference between a smooth launch and a frustrating one.