I get the following warning:
Status: failed
Processing content.opf...
ibooks:
http://vocabulary.itunes.apple.com/r...xtensions-1.0/ package prefix attribute deleted.
Package updated.
Traceback (most recent call last):
File "D:\Program Files\Sigil\plugin_launchers\python\launcher.py", line 135, in launch
self.exitcode = target_script.run(container)
File "C:\Users\boeke\AppData\Local\sigil-ebook\sigil\plugins\ePub2\plugin.py", line 207, in run
timestamp.string = timestamp.string[0:10]
AttributeError: 'NoneType' object has no attribute 'string'
Error: 'NoneType' object has no attribute 'string'