Gftools builder: AttributeError: 'FoundANone' object has no attribute 'argv'

I'm trying to export a font which is quite simple (one weight axes, a regular and an italic), and I still have this error when I use gftools builder. You can find the files here: [[https://github.com/emmamarichal/Familjen-Grotesk]](https://github.com/emmamarichal/Familjen-Grotesk)
Last commit: [24fd988](https://github.com/emmamarichal/Familjen-Grotesk/commit/24fd98845b6d4720d7ec240bddbd5d54cf72381b)

Thanks :)
cc @RosaWagner

### Error report
I run `gftools builder config.yaml`

INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:fontmake.font_project:Building variable font ../fonts/variable/FamiljenGrotesk-VF.ttf
Traceback (most recent call last):
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontmake/font_project.py", line 968, in run_from_designspace
self._run_from_designspace_interpolatable(
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontmake/font_project.py", line 1048, in _run_from_designspace_interpolatable
self.build_variable_font(
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontmake/font_project.py", line 339, in build_variable_font
font = ufo2ft.compileVariableTTF(
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/ufo2ft/__init__.py", line 554, in compileVariableTTF
varfont = varLib.build(
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/__init__.py", line 926, in build
_merge_OTL(vf, model, master_fonts, axisTags)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/__init__.py", line 619, in _merge_OTL
merger.mergeTables(font, master_fonts, ['GSUB', 'GDEF', 'GPOS'])
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 139, in mergeTables
self.mergeThings(font[tag], [m[tag] if tag in m else None
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 1044, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 127, in mergeThings
self.mergeObjects(out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 102, in mergeObjects
mergerFunc(self, value, values)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 1044, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 127, in mergeThings
self.mergeObjects(out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 102, in mergeObjects
mergerFunc(self, value, values)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 1044, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 127, in mergeThings
self.mergeObjects(out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 102, in mergeObjects
mergerFunc(self, value, values)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 1044, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 129, in mergeThings
self.mergeLists(out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 112, in mergeLists
self.mergeThings(value, values)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 1044, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 125, in mergeThings
mergerFunc(self, out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 833, in merge
merger.mergeLists(self.SubTable, subtables)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 112, in mergeLists
self.mergeThings(value, values)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 1044, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 125, in mergeThings
mergerFunc(self, out, lst)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 267, in merge
merger.mergeThings(self.Value, [v.Value for v in lst])
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/merger.py", line 1038, in mergeThings
raise FoundANone(self, got=lst)
fontTools.varLib.errors.FoundANone:

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/gftools/builder/__init__.py", line 289, in build_variable
output_files = self.run_fontmake(source, args)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/gftools/builder/__init__.py", line 329, in run_fontmake
FontProject().run_from_glyphs(source, **args)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontmake/font_project.py", line 719, in run_from_glyphs
self.run_from_designspace(designspace_path, **kwargs)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontmake/font_project.py", line 982, in run_from_designspace
raise FontmakeError(
fontmake.errors.FontmakeError:

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/Users/marichalemma/Google/env/bin/gftools-builder.py", line 83, in
builder.build()
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/gftools/builder/__init__.py", line 186, in build
self.build_variable()
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/gftools/builder/__init__.py", line 294, in build_variable
self.logger.error("Could not build variable font: %s" % e)
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontmake/errors.py", line 33, in __str__
cause = str(self.__cause__) if self.__cause__ is not None else None
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/errors.py", line 61, in __str__
offender_index, offender = self.offender
File "/Users/marichalemma/Google/env/lib/python3.9/site-packages/fontTools/varLib/errors.py", line 105, in offender
cause = self.argv[0]
AttributeError: 'FoundANone' object has no attribute 'argv'