"Compiling ufo failed" Error when generating a static font

This error was reported when generating a static font. I never had it before so I don't know what it means:

`fontmake.errors.FontmakeError: In 'Festive-Pro.glyphs' -> 'master_ufo/FestivePro-Regular.designspace' -> 'master_ufo/instance_ufo/FestivePro-Regular.ufo': Compiling UFO failed: not enough values to unpack (expected 4, got 2)`

Glyphs file: [https://github.com/googlefonts/festive/blob/master/sources/Festive-Pro.glyphs](https://github.com/googlefonts/festive/blob/master/sources/Festive-Pro.glyphs)

Command used: `gftools builder config.yml` and this one to verify it `fontmake -g Festive-Pro.glyphs -i -o ttf`