Error when generating variable fonts

Hello,
I have this variable font where names represent degree of rotation. When I put zero, it doesn't work and is recognized as a byte? Is there a workaround?

tested with latest fontmake and 2.0.0

```fontmake -s -m "rotor_360_underlay.designspace" -o variable --output-path exports/full/VF/rotor-VF.ttf --verbose DEBUG```
```





```

```
Traceback (most recent call last):
File "/Users/jansindler/.pyenv/versions/3.7.4/bin/fontmake", line 8, in
sys.exit(main())
File "/Users/jansindler/.pyenv/versions/3.7.4/lib/python3.7/site-packages/fontmake/__main__.py", line 424, in main
project.run_from_designspace(designspace_path, **args)
File "/Users/jansindler/.pyenv/versions/3.7.4/lib/python3.7/site-packages/fontmake/font_project.py", line 873, in run_from_designspace
**kwargs,
File "/Users/jansindler/.pyenv/versions/3.7.4/lib/python3.7/site-packages/fontmake/font_project.py", line 939, in _run_from_designspace_interpolatable
designspace, output_path=output_path, output_dir=output_dir, **kwargs
File "/Users/jansindler/.pyenv/versions/3.7.4/lib/python3.7/site-packages/fontmake/font_project.py", line 299, in build_variable_font
inplace=True,
File "/Users/jansindler/.pyenv/versions/3.7.4/lib/python3.7/site-packages/ufo2ft/__init__.py", line 608, in compileVariableTTF
ttfDesignSpace, exclude=excludeVariationTables, optimize=optimizeGvar
File "/Users/jansindler/.pyenv/versions/3.7.4/lib/python3.7/site-packages/fontTools/varLib/__init__.py", line 887, in build
fvar = _add_fvar(vf, ds.axes, ds.instances)
File "/Users/jansindler/.pyenv/versions/3.7.4/lib/python3.7/site-packages/fontTools/varLib/__init__.py", line 96, in _add_fvar
inst.subfamilyNameID = nameTable.addMultilingualName(localisedStyleName)
File "/Users/jansindler/.pyenv/versions/3.7.4/lib/python3.7/site-packages/fontTools/ttLib/tables/_n_a_m_e.py", line 273, in addMultilingualName
names, windows=windows, mac=mac, minNameID=minNameID)
File "/Users/jansindler/.pyenv/versions/3.7.4/lib/python3.7/site-packages/fontTools/ttLib/tables/_n_a_m_e.py", line 229, in findMultilingualName
key = (name.toUnicode(), name.platformID,
File "/Users/jansindler/.pyenv/versions/3.7.4/lib/python3.7/site-packages/fontTools/ttLib/tables/_n_a_m_e.py", line 456, in toUnicode
string = b'\0' + string
TypeError: can't concat str to bytes
```