new fontmake behaviour: AssertionError while compiling src/NotoSansCarian-MM.glyphs

source: https://github.com/googlei18n/noto-source/blob/master/src/NotoSansCarian-MM.glyphs

this is one of the fonts that has only Regular weight/width.

however, previous fontmake would build NotoSansCarian-Regular-VF.ttf

now it gets assertion error.

If this is an intended behaviour I'd like to know what needs to change in the sources to produce a font

fontmake -g src/NotoSansCarian-MM.glyphs -o variable
INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:glyphsLib:Parsing .glyphs file
INFO:glyphsLib:Casting parsed values
INFO:glyphsLib:Loading to UFOs
INFO:glyphsLib.util:Writing master_ufo/NotoSansCarian-Regular.ufo
INFO:fontmake.font_project:Building interpolation-compatible TTFs
INFO:fontmake.font_project:Converting curves compatibly
INFO:cu2qu.ufo:New spline lengths:
1: 20
2: 144
3: 4

INFO:fontmake.font_project:Saving TTF for NotoSansCarian-Regular
INFO:fontmake.font_project:Building variable font variable_ttf/NotoSansCarian-Regular-VF.ttf
INFO:fontTools.varLib:Internal master locations:
[{}]
INFO:fontTools.varLib:Internal axis supports:
{}
INFO:fontTools.varLib:Normalized master locations:
[{}]
INFO:fontTools.varLib:Index of base master: 0
INFO:fontTools.varLib:Building variable font
INFO:fontTools.varLib:Loading master fonts
Traceback (most recent call last):
File "/usr/local/bin/fontmake", line 9, in
load_entry_point('fontmake==1.2.4.dev0', 'console_scripts', 'fontmake')()
File "/usr/local/google/home/marekj/newrel/fontmake/Lib/fontmake/__main__.py", line 182, in main
project.run_from_glyphs(glyphs_path, **args)
File "/usr/local/google/home/marekj/newrel/fontmake/Lib/fontmake/font_project.py", line 379, in run_from_glyphs
designspace_path, instance_data=instance_data, **kwargs)
File "/usr/local/google/home/marekj/newrel/fontmake/Lib/fontmake/font_project.py", line 435, in run_from_designspace
interpolate_layout_from=interpolate_layout_from, **kwargs)
File "/usr/local/google/home/marekj/newrel/fontmake/Lib/fontmake/font_project.py", line 495, in run_from_ufos
self.build_variable_font(designspace_path)
File "/usr/local/google/home/marekj/newrel/fontmake/Lib/fontmake/font_project.py", line 228, in build_variable_font
font, _, _ = varLib.build(designspace_path, finder)
File "/usr/local/google/home/marekj/newrel/fonttools/Lib/fontTools/varLib/__init__.py", line 558, in build
fvar,avar = _add_fvar_avar(vf, axes, instances)
File "/usr/local/google/home/marekj/newrel/fonttools/Lib/fontTools/varLib/__init__.py", line 58, in _add_fvar_avar
assert axes
AssertionError