My first attempt at extending next sever's babel config (https://nextjs.org/docs/advanced-features/customizing-babel-config) with seemingly the appropriate plugin (https://babeljs.io/docs/en/babel-plugin-proposal-class-properties) didn't work; I can see the next server picking the babel config file up, but I get the same error. Could be that webpack loaders needs some special coercing on top of that based on the message, though.