Best way to add empty kerning to a UFO (to make compatible with kerned font)?

[ArrowType]

I'm adding blank kerning to a monospace font, in order to make it compatible with a proportional font. Ultimately, I'm building a variable font with a "Monospace" axis, via FontMake, and it fails if some sources have kerning, but others don't.

Is there a better way to add blank kerning than this?

if len(f.kerning) == 0:
    f.kerning[("A", "A")] = 0

[frederik]

fontMake requires all masters have the same kerning pairs.

Batch is solving this for you by adding interpolated values for non existing pairs

f.kerning[("A", "A")] = 0 is the only way

good luck