Problem generating feature variations

Hello, I am getting this error when exporting a variable font: Core: Problem generating feature variations.
No extra info is provided. What does this mean, and how can I fix it? Thanks!

Can you send me that file?

Okay, I think I have found the issue. In the file, some instances had the same names. If they all have different names, it works.

The file you send me, some instances had no name at all. I’ll try to add a better error message for both cases.