Compatibility problem when exporting Variable font

I have already managed to create one variable font family, but I get a problem when trying to export a different font:

Can’t convert to compatible TrueType curves

Glyph ‘c_h.end’ is not compatible.

I can’t see what the problem is as it exports fine as single weights.

Two possible scenarios:

  1. in the single weights, are all instances actually interpolations between two masters? Or is there a third master that sometimes is not compatible to the other two? In a static font export, it wouldn’t matter, in a variable font, it does.
  2. there is a curve segment that bends very differently between masters, which causes the PS-to-TT conversion to fail. In that case, we be interested in the outlines.

It’s a four weight script font. The two intermediate weights are interpolated instances added as Masters. I delete the two intermediate masters, fix the incompatible glyph in the two extreme masters then add the two intermediate instances as masters and export a variable font, only for another incompatible warning to pop up. Is there any way a list of incompatibles could appear instead of one at a time? Nothing shows as incompatible in the font view.

Can you add a Force Compatibility Check custom parameter to the font?

And why did you add the intermediate masters? Did you change them a lot?

Oh yes, I forgot I don’t need to add intermediate masters if nothing changes between the two extremes. I was still influenced by the previous variable that I generated.
It’s all fixed now. Many thanks to both of you. Amazing support for this excellent app. I love it.