Incompatible masters found in varLib._add_gvar()

I’ve ran into this traceback,

Traceback (most recent call last):
  File "/fontTools/varLib/__init__.py", line 633, in build
    _add_gvar(vf, model, master_fonts)
  File "/fontTools/varLib/__init__.py", line 248, in _add_gvar
    allCoords = [d[0] for d in allData]
TypeError: 'NoneType' object has no attribute '__getitem__'

It was caused by U+0020 being named space in one master font, and uni0020 in the other.

@behdad where should a check for this kind of incompatibility be added?

[behdad]

I think all that’s needed is a better error msg.

No, long-term I do want to allow masters that do not have all glyphs. Aka the brace trick: https://glyphsapp.com/tutorials/additional-masters-for-individual-glyphs-the-brace-trick

[justvanrossum]

Makes sense. But Miguel’s case is clearly a mistake, where two glyphs with the same code point have different glyph names.