OverflowError: unsigned byte integer is greater than maximum

I’m getting the following error when trying to compile a variable font. This has been happening to all of the UFOs in the project, across 4 distinct .designspaces. For one of them, renaming commaaccentabove to commaaccent.alt seemed to inexplicably solve the issue, but for the other variable fonts, I’m unable to solve the issue without deleting half of the character-set (doesn’t seem to matter which half). Please advise!

File "/Applications/RoboFont-4_2b2.app/Contents/Resources/lib/python3.7/fontTools/ttLib/tables/_c_m_a_p.py", line 176, in compile
KeyError: 4769113184

During handling of the above exception, another exception occurred:

    Traceback(most recent call last):
    File "/Applications/RoboFont-4_2b2.app/Contents/Resources/lib/python3.7/fontCompiler/ttfCompiler.py", line 599, in _makefont
File "/Applications/RoboFont-4_2b2.app/Contents/Resources/lib/python3.7/fontTools/ttLib/ttFont.py", line 181, in save
File "/Applications/RoboFont-4_2b2.app/Contents/Resources/lib/python3.7/fontTools/ttLib/ttFont.py", line 221, in _save
File "/Applications/RoboFont-4_2b2.app/Contents/Resources/lib/python3.7/fontTools/ttLib/ttFont.py", line 644, in _writeTable
File "/Applications/RoboFont-4_2b2.app/Contents/Resources/lib/python3.7/fontTools/ttLib/ttFont.py", line 666, in getTableData

OverflowError: unsigned byte integer is greater than maximum 

Lines in question: https://github.com/fonttools/fonttools/blob/e92dd05101e58a4abdf7f179a25f2d19f9a97d8b/Lib/fontTools/ttLib/tables/_c_m_a_p.py#L176 https://github.com/fonttools/fonttools/blob/e92dd05101e58a4abdf7f179a25f2d19f9a97d8b/Lib/fontTools/ttLib/ttFont.py#L666

Is it a problem with my glyph names, or order, or something else?

[justvanrossum]

(probably messing up the metaphor, but I’m sure you know what I mean…)

[justvanrossum]

Or: abandon FDK and move to fontmake.

I’m personally not touching AFDKO with a 10 foot pole, and I couldn’t be happier.