 "." is not a valid export - TypeError

In Firefox: TypeError: ({}) is not a function 
In Chrome: TypeError: Chessboard is not a function
In jspm online generator (https://generator.jspm.io): "." is not a valid export

I've been working in Rails 7 trying to pin (Importmap) your npm package.
I think you should consider to add something like: "main": "lib/chessboard.js" to your package.json

Could you check it?

Thanks,