by Kevin Siegel 
I write and produce all of the print versions of my "skills & drills" workbooks in Adobe FrameMaker. When it comes to creating digital versions of my books, I take the FrameMaker content into Adobe RoboHelp and then generate both ePubs and MOBI outputs. The whole process of going from FrameMaker to RoboHelp to eBook takes just a few moments. I've written about the workflow previously.
Normally my eBooks generate without issue. However, using the new Adobe RoboHelp 2015, I started receiving the following error message when generating my eBooks:
Validating against EPUB version 3.0
September 2, 2015 1:05:34 PM EDT
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ERROR: RoboHelp2105.epub/OEBPS/__toc.ncx: External DTD entities are not allowed. Remove the DOCTYPE.
Check finished with warnings or errors!
I researched the error and it turns out the ePub 3 specifications do not accept references to DTD entities. RoboHelp was adding the reference automatically every time I generated. Because RoboHelp does not have an option to remove the DOCTYPE code from the eBook output, I was stuck. Thankfully, a quick email to my contacts at Adobe led to an easy fix that resolved the issue. Special thanks to Adobe's Amit Jha for the following:
- Using Windows Explorer, open the folder where RoboHelp has been installed on your computer. (For me, this was C:\Program Files\Adobe\Adobe RoboHelp 2015\RoboHTML.)
- Open EBooksExt\EPUB3\OEBPS\
- Open the file named _toc.ncx. (It will open automatically in Notepad.)
- Remove the following line of code from the file:
<!DOCTYPE ncx PUBLIC "-//NISO//DTD ncx 2005-1//EN" "http://www.daisy.org/z3986/2005/ncx-2005-1.dtd">
- Save and close the _toc.ncx file.
- Back in RoboHelp, generate the eBook layout.
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