Originally Posted by Jellby
Some devices (older Sonys?) might choke on such an ePub, refuse to open it or even crash.
If that's an acceptable risk for you, you can ignore the error, as the ePub is valid anyway. But if you want to make sure the book works as good as possible in all devices, you better split the file.
Thanks for the reply. I thought this might be the case. There is a Part Title about halfway through the book, and will be a convenient split.