The common stacktrace like this:
File "fonttools/Lib/fontTools/ttLib/__init__.py", line 202, in save
self._writeTable(tag, writer, done)
File "fonttools/Lib/fontTools/ttLib/__init__.py", line 631, in _writeTable
tabledata = self.getTableData(tag)
File "fonttools/Lib/fontTools/ttLib/__init__.py", line 644, in getTableData
return self.tables[tag].compile(self)
File "fonttools/Lib/fontTools/ttLib/tables/otBase.py", line 86, in compile
self.table.compile(writer, font)
File "fonttools/Lib/fontTools/ttLib/tables/otBase.py", line 681, in compile
conv.write(writer, font, table, value)
File "fonttools/Lib/fontTools/ttLib/tables/otConverters.py", line 354, in write
value.compile(subWriter, font)
File "fonttools/Lib/fontTools/ttLib/tables/otBase.py", line 661, in compile
conv.write(writer, font, table, value, i)
File "fonttools/Lib/fontTools/ttLib/tables/otConverters.py", line 354, in write
value.compile(subWriter, font)
File "fonttools/Lib/fontTools/ttLib/tables/otBase.py", line 661, in compile
conv.write(writer, font, table, value, i)
File "fonttools/Lib/fontTools/ttLib/tables/otConverters.py", line 354, in write
value.compile(subWriter, font)
File "fonttools/Lib/fontTools/ttLib/tables/otBase.py", line 661, in compile
conv.write(writer, font, table, value, i)
File "fonttools/Lib/fontTools/ttLib/tables/otConverters.py", line 277, in write
value.compile(writer, font)
File "fonttools/Lib/fontTools/ttLib/tables/otBase.py", line 681, in compile
conv.write(writer, font, table, value)
File "fonttools/Lib/fontTools/ttLib/tables/otConverters.py", line 175, in write
writer.writeUShort(value)
File "fonttools/Lib/fontTools/ttLib/tables/otBase.py", line 457, in writeUShort
assert 0 <= value < 0x10000, value
AssertionError: None
now has this as the last line:
AssertionError: (None, 'LookupListIndex', 'SubstLookupRecord', 'ChainContextSubst', 'Lookup', 'LookupList')
which means a value of None was tried for writing a LookupListIndex from a ChainContextSubset...
It's a hack, but a very useful one.
1fbad696c9