Generating fonts from Designspace failed: ('an integer is required (got type NoneType)', ...

Hi, I'm experiencing an inability to generate a variable font with `fontmake [designspace file] -o variable`. I'm getting the following error that I don't know how to interpret.

```
...
INFO:fontTools.varLib:Generating HVAR
INFO:fontTools.varLib:Merging OpenType Layout tables
INFO:fontTools.varLib:Generating gvar
INFO:fontTools.varLib:Merging TT hinting
INFO:fontTools.varLib:Generating GSUB FeatureVariations
INFO:fontTools.varLib:Setting OS/2.usWeightClass = 1
fontmake: Error: In '[path to designspace file]': Generating fonts from Designspace failed: ('an integer is required (got type NoneType)', 'Substitute[]', 0, 'SubTable[]', 1, 'Lookup[]', 'LookupList')
```

- It's a 4-source designspace.
- All of the anchors are the same.
- Glyph compatibility was passed.
- Nothing is reported in designspaceProblems.
- Unicodes seems to be all good.
- Glyph order the same across all sources.

I'm stumped! Any leads would be greatly appreciated.