gftools: gftools-add-font.py: Not working with variable fonts

gftools-add-font.py works for static fonts, for the root directory of gftools I do the following:

python2 bin/gftools-add-font.py ~/Google/fonts/ofl/staatliches

And get the result:

Wrote /Users/Air/Google/fonts/ofl/font/DESCRIPTION.en_us.html
Wrote /Users/Air/Google/fonts/ofl/font/METADATA.pb

But when I run the same command from the same directory on a variable font, I get the following traceack:

Traceback (most recent call last):
  File "bin/gftools-add-font.py", line 267, in <module>
    app.run()
  File "/usr/local/Cellar/python@2/2.7.15_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/google/apputils/app.py", line 238, in run
    return _actual_start()
  File "/usr/local/Cellar/python@2/2.7.15_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/google/apputils/app.py", line 267, in _actual_start
    really_start()
  File "/usr/local/Cellar/python@2/2.7.15_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/google/apputils/app.py", line 220, in really_start
    sys.exit(main(argv))
  File "bin/gftools-add-font.py", line 253, in main
    metadata = _MakeMetadata(fontdir, is_new)
  File "bin/gftools-add-font.py", line 114, in _MakeMetadata
    file_family_style_weights = _FileFamilyStyleWeights(fontdir)
  File "bin/gftools-add-font.py", line 89, in _FileFamilyStyleWeights
    result = [fonts.FileFamilyStyleWeight(f) for f in files]
  File "/Users/Air/Demo/gftools/Lib/gftools/util/google_fonts.py", line 462, in FileFamilyStyleWeight
    sw = StyleWeight(m.group(2))
  File "/Users/Air/Demo/gftools/Lib/gftools/util/google_fonts.py", line 446, in StyleWeight
    return ('normal', _KNOWN_WEIGHTS[styleweight])
KeyError: 'VF'