Not sure the best way to fix it if it's an lnbits mint, this info endpoint should return a public key for the mint.
https://obsessedjerky7.lnbits.com/cashu/api/v1/L4bNfiBtTyhEpZWdt6QTb4/v1/info
We still add the mint, but just point it out to try to get you to follow the spec.