[GeckoFriend]

I tried to split an epub today and received the following error message:

calibre, version 3.3.0
ERROR: Unhandled exception: <b>AttributeError</b>:'NoneType' object has no attribute 'getElementsByTagName'

calibre 3.3 embedded-python: True is64bit: False
Windows-10-10.0.15063-SP0 Windows ('32bit', 'WindowsPE')
32bit process running on 64bit windows
('Windows', '10', '10.0.15063')
Python 2.7.12+
Windows: ('10', '10.0.15063', 'SP0', u'Multiprocessor Free')
Interface language: None
Successfully initialized third party plugins: DeDRM (6, 3, 2) && Kindle and Mobipocket DeDRM (0, 4, 18) && Ignoble Epub DeDRM (0, 2, 6) && Inept Epub DeDRM (0, 2, 1) && Inept PDF DeDRM (0, 1, 9) && eReader PDB 2 PML (0, 0, 8) && Modify ePub (1, 3, 13) && Barnes & Noble (1, 2, 14) && EpubMerge (2, 1, 1) && Reading List (1, 6, 6) && Find Duplicates (1, 6, 3) && Manage Series (1, 2, 8) && Overdrive Link (2, 15, 0) && Open With (1, 5, 10) && Count Pages (1, 8, 0) && Favourites Menu (1, 0, 4) && Search The Internet (1, 7, 6) && Quality Check (1, 9, 11) && View Manager (1, 4, 3) && EpubSplit (2, 2, 1) && Resize Cover (1, 0, 2) && Overdrive Link* (2, 15, 0)
Traceback (most recent call last):
File "calibre_plugins.epubsplit.epubsplit_plugin", line 137, in plugin_button
File "calibre_plugins.epubsplit.epubsplit", line 624, in get_split_lines
File "calibre_plugins.epubsplit.epubsplit", line 556, in get_toc_map
AttributeError: 'NoneType' object has no attribute 'getElementsByTagName'

I have no clue what this means. Any help greatly appreciated.