fontmake.instantiator.InstantiatorError: Can't generate UFOs from this designspace: no default font.

I have a [Glyphs source](https://github.com/yanone/Sen/blob/master/Glyphs/sen.glyphs) here that should easily compile into fonts with `fontmake` as far as I'm concerned.

It's the most simple setup: Two master, three instances. Yet, when I build using `fontmake -g ./Glyphs/sen.glyphs -o ttf -i --output-dir $TTFDIR` (taken from build script), the following traceback appears:

```
Traceback (most recent call last):
File "/usr/local/bin/fontmake", line 8, in
sys.exit(main())
File "/usr/local/lib/python3.7/site-packages/fontmake/__main__.py", line 425, in main
project.run_from_glyphs(glyphs_path, **args)
File "/usr/local/lib/python3.7/site-packages/fontmake/font_project.py", line 672, in run_from_glyphs
self.run_from_designspace(designspace_path, **kwargs)
File "/usr/local/lib/python3.7/site-packages/fontmake/font_project.py", line 889, in run_from_designspace
**kwargs,
File "/usr/local/lib/python3.7/site-packages/fontmake/font_project.py", line 932, in _run_from_designspace_static
expand_features_to_instances=expand_features_to_instances,
File "/usr/local/lib/python3.7/site-packages/fontmake/font_project.py", line 707, in interpolate_instance_ufos
designspace, round_geometry=round_instances
File "/usr/local/lib/python3.7/site-packages/fontmake/instantiator.py", line 185, in from_designspace
"Can't generate UFOs from this designspace: no default font."
fontmake.instantiator.InstantiatorError: Can't generate UFOs from this designspace: no default font.
```

What am I doing wrong?