ufo2ft.errors.InvalidDesignSpaceData: Can't find base (neutral) master in DesignSpace document

I am using fontmake 2.0.8, and when I try to generate a vf using the following command on a .glyphs file:

`fontmake -o variable -g Error-Upright.glyphs `

I get these warnings:

```
WARNING:glyphsLib.builder.axes:Instance user-space location (100) redefined by 'Condensed UltraLight'
WARNING:glyphsLib.builder.axes:Instance user-space location (400) redefined by 'Condensed Regular'
WARNING:glyphsLib.builder.axes:Instance user-space location (900) redefined by 'Condensed UltraBlack'
WARNING:glyphsLib.builder.axes:Instance user-space location (100) redefined by 'SemiCondensed UltraLight'
WARNING:glyphsLib.builder.axes:Instance user-space location (400) redefined by 'SemiCondensed Regular'
WARNING:glyphsLib.builder.axes:Instance user-space location (100) redefined by 'UltraLight'
WARNING:glyphsLib.builder.axes:Instance user-space location (400) redefined by 'Regular'
WARNING:glyphsLib.builder.axes:Instance user-space location (100) redefined by 'Extended UltraLight'
WARNING:glyphsLib.builder.axes:Instance user-space location (400) redefined by 'Extended Regular'
WARNING:glyphsLib.builder.axes:Instance user-space location (100) redefined by 'ExtraExtended UltraLight'
```

I am guessing that these warnings are probably to do with the instances not being orthogonal to each other? @twardoch [has suggested](https://github.com/daltonmaag/statmake/issues/19#issuecomment-573628557) that to solve that particular problem the instances need to be generated as masters. However that's not a realistic solution as there probably needs a dozen or so instances to be generate, this is the design space:


The last error is:

`ufo2ft.errors.InvalidDesignSpaceData: Can't find base (neutral) master in DesignSpace document`

I have a `Variable Font Origin` custom parameter set for this font. The font generates just fine without the instances though. Is it to do with the above issue?

Please see the sample project here
[Error-Upright.glyphs.zip](https://github.com/googlefonts/fontmake/files/4063445/Error-Upright.glyphs.zip)