Convert Epub to Kepub

[Quoth]

2 out of 38 failed. It claimed at first a deDRM error, which was nonsense as all sources were DRM free (my own files from docx).
So disabled deDrm plugin.
Failed again!

Convert book 1 of 1 (The Seven Talismans (v12))
Conversion options changed from defaults:
  margin_bottom: 0.0
  read_metadata_from_opf: '/tmp/calibre_5.40.0_tmp_wgt6kk_9/f1376ahq.opf'
  minimum_line_height: 0.0
  kepub_hyphenate: False
  embed_all_fonts: True
  remove_fake_margins: False
  expand_css: True
  subset_embedded_fonts: True
  margin_top: 0.0
  output_profile: 'tablet'
  cover: '/tmp/calibre_5.40.0_tmp_wgt6kk_9/dk7d3m33.jpeg'
  keep_ligatures: True
  verbose: 2
Resolved conversion options
calibre version: 5.40.0
{'asciiize': False,
 'author_sort': None,
 'authors': None,
 'base_font_size': 0.0,
 'book_producer': None,
 'change_justification': 'original',
 'chapter': "//*[((name()='h1' or name()='h2') and re:test(., "
            "'\\s*((chapter|book|section|part)\\s+)|((prolog|prologue|epilogue)(\\s+|$))', "
            "'i')) or @class = 'chapter']",
 'chapter_mark': 'pagebreak',
 'comments': None,
 'cover': '/tmp/calibre_5.40.0_tmp_wgt6kk_9/dk7d3m33.jpeg',
 'debug_pipeline': None,
 'dehyphenate': True,
 'delete_blank_paragraphs': True,
 'disable_font_rescaling': False,
 'dont_split_on_page_breaks': False,
 'duplicate_links_in_toc': False,
 'embed_all_fonts': True,
 'embed_font_family': None,
 'enable_heuristics': False,
 'epub_flatten': False,
 'epub_inline_toc': False,
 'epub_toc_at_end': False,
 'epub_version': '3',
 'expand_css': True,
 'extra_css': None,
 'extract_to': None,
 'filter_css': '',
 'fix_indents': True,
 'flow_size': 260,
 'font_size_mapping': None,
 'format_scene_breaks': True,
 'html_unwrap_factor': 0.4,
 'input_encoding': None,
 'input_profile': <calibre.customize.profiles.InputProfile object at 0x7f815b32ba90>,
 'insert_blank_line': False,
 'insert_blank_line_size': 0.5,
 'insert_metadata': False,
 'isbn': None,
 'italicize_common_cases': True,
 'keep_ligatures': True,
 'kepub_clean_markup': True,
 'kepub_disable_hyphenation': False,
 'kepub_hyphenate': False,
 'kepub_hyphenate_chars': 6,
 'kepub_hyphenate_chars_after': 3,
 'kepub_hyphenate_chars_before': 3,
 'kepub_hyphenate_limit_lines': 2,
 'language': None,
 'level1_toc': None,
 'level2_toc': None,
 'level3_toc': None,
 'line_height': 0.0,
 'linearize_tables': False,
 'margin_bottom': 0.0,
 'margin_left': 5.0,
 'margin_right': 5.0,
 'margin_top': 0.0,
 'markup_chapter_headings': True,
 'max_toc_links': 50,
 'minimum_line_height': 0.0,
 'no_chapters_in_toc': False,
 'no_default_epub_cover': False,
 'no_inline_navbars': False,
 'no_svg_cover': False,
 'output_profile': <calibre.customize.profiles.TabletOutput object at 0x7f815b2bc460>,
 'page_breaks_before': '/',
 'prefer_metadata_cover': False,
 'preserve_cover_aspect_ratio': False,
 'pretty_print': True,
 'pubdate': None,
 'publisher': None,
 'rating': None,
 'read_metadata_from_opf': '/tmp/calibre_5.40.0_tmp_wgt6kk_9/f1376ahq.opf',
 'remove_fake_margins': False,
 'remove_first_image': False,
 'remove_paragraph_spacing': False,
 'remove_paragraph_spacing_indent_size': 1.5,
 'renumber_headings': True,
 'replace_scene_breaks': '',
 'search_replace': '[]',
 'series': None,
 'series_index': None,
 'smarten_punctuation': False,
 'sr1_replace': None,
 'sr1_search': None,
 'sr2_replace': None,
 'sr2_search': None,
 'sr3_replace': None,
 'sr3_search': None,
 'start_reading_at': None,
 'subset_embedded_fonts': True,
 'tags': None,
 'timestamp': None,
 'title': None,
 'title_sort': None,
 'toc_filter': None,
 'toc_threshold': 6,
 'toc_title': None,
 'transform_css_rules': '[]',
 'transform_html_rules': '[]',
 'unsmarten_punctuation': False,
 'unwrap_lines': True,
 'use_auto_toc': False,
 'verbose': 2}
Failed to initialize plugin: '/home/mikew/.config/calibre/plugins/FictionDB.zip'
Failed to initialize plugin: '/home/mikew/.config/calibre/plugins/K4PC, K4Mac, Kindle Mobi and Topaz DeDRM.zip'
Failed to initialize plugin: '/home/mikew/.config/calibre/plugins/Kindle covers (G).zip'
InputFormatPlugin: EPUB Input running
on /tmp/calibre_5.40.0_tmp_wgt6kk_9/6wccffn9.epub
Found HTML cover titlepage.xhtml
Parsing all content...
Parsing index_split_006.html ...
Parsing index_split_020.html ...
Parsing index_split_046.html ...
Parsing index_split_001.html ...
Parsing index_split_015.html ...
Parsing index_split_031.html ...
Parsing index_split_041.html ...
Parsing index_split_026.html ...
Parsing index_split_036.html ...
Parsing index_split_052.html ...
Parsing index_split_047.html ...
Parsing index_split_007.html ...
Parsing index_split_021.html ...
Parsing index_split_002.html ...
Parsing index_split_016.html ...
Parsing index_split_032.html ...
Parsing index_split_042.html ...
Parsing index_split_027.html ...
Parsing index_split_037.html ...
Parsing index_split_038.html ...
Parsing index_split_053.html ...
Parsing index_split_008.html ...
Parsing index_split_022.html ...
Parsing index_split_048.html ...
Parsing index_split_003.html ...
Parsing index_split_017.html ...
Parsing index_split_033.html ...
Parsing index_split_043.html ...
Parsing page_styles.css ...
Parsing index_split_012.html ...
Parsing index_split_028.html ...
Parsing index_split_009.html ...
Parsing index_split_023.html ...
Parsing index_split_049.html ...
Parsing index_split_004.html ...
Parsing index_split_018.html ...
Parsing index_split_034.html ...
Parsing index_split_044.html ...
Parsing stylesheet.css ...
Parsing titlepage.xhtml ...
Parsing index_split_013.html ...
Parsing index_split_029.html ...
Parsing index_split_039.html ...
Parsing index_split_010.html ...
Parsing index_split_024.html ...
Parsing index_split_050.html ...
Parsing index_split_005.html ...
Parsing index_split_019.html ...
Parsing index_split_035.html ...
Parsing index_split_045.html ...
Parsing index_split_000.html ...
Parsing index_split_014.html ...
Parsing index_split_030.html ...
Parsing index_split_040.html ...
Parsing index_split_051.html ...
Parsing index_split_011.html ...
Parsing index_split_025.html ...
Reading TOC from NCX...
Merging user specified metadata...
Detecting structure...
Flattening CSS and remapping font sizes...
Source base font size is 12.00000pt
Failed to find fonts for family: Courier not embedding
Decreased the font fonts/Droid Sans - Regular.ttf to 47.3% of its original size
Decreased the font fonts/Georgia - Bold.ttf to 24.2% of its original size
Decreased the font fonts/Droid Sans - Bold.ttf to 46.1% of its original size
Decreased the font fonts/Georgia - Regular.ttf to 37.4% of its original size
The font fonts/Georgia.ttf is unused. Removing it.
The font fonts/Georgia-Bold.ttf is unused. Removing it.
The font fonts/Georgia-Italic.ttf is unused. Removing it.
The font fonts/Georgia-Bold-Italic.ttf is unused. Removing it.
Decreased the font fonts/Alois-Heavy.ttf to 38.0% of its original size
Traceback (most recent call last):
  File "runpy.py", line 194, in _run_module_as_main
  File "runpy.py", line 87, in _run_code
  File "site.py", line 45, in <module>
  File "site.py", line 41, in main
  File "calibre/utils/ipc/worker.py", line 215, in main
  File "calibre/gui2/convert/gui_conversion.py", line 38, in gui_convert_override
  File "calibre/gui2/convert/gui_conversion.py", line 25, in gui_convert
  File "calibre/ebooks/conversion/plumber.py", line 1256, in run
  File "calibre/ebooks/oeb/transforms/subset.py", line 159, in __call__
  File "calibre/utils/fonts/sfnt/subset.py", line 130, in subset
  File "calibre/utils/fonts/sfnt/container.py", line 52, in __init__
  File "calibre/utils/fonts/sfnt/kern.py", line 21, in __init__
struct.error: unpack_from requires a buffer of at least 4 bytes for unpacking 4 bytes at offset 0 (actual buffer size is 0)

I disabled Subset fonts and it worked. The fonts normally are already subsetted, but perhaps weren't on the two that failed.