Thank you for this! The IDPF validator has a 10MB limit and my file (a children's picture book) is much larger. Got an error message from IDPF. Thoughts?
If you are hoping to convert this ePub to mobi format to sell via Amazon, have a look at their spec to check out the implications of the size.
It might be worth taking out a month's worth of Flightdeck to check your ePub in detail.
Are you on a Mac?
I had this same issue and it is because you need to export a Table of Contents with the file.
Make sure to select a Navigation TOC (even if the book doesn't have one) and then select the default TOC style.
Is the "Navigation TOC" something I choose as I am creating the ePub file in inDesign or is it something I select in Document Setup?
It is during the EPUB export process in the General section. The default has nothing selected.