Why does winston output undefined?

Question

Node.js

let winston = require("winston");

let logger = winston.createLogger({
    format: {
        transform(info, opts) {
            info.message = "!" + info.message + "!";
            return info;
        }
    },
    transports: [
        new winston.transports.Console()
    ]
});

logger.info("Something");

The code outputs "undefined". Why is this happening?
When choosing built-in formats, everything works fine. But when I write my own, or even paste the format from the examples from the official winston documentation, then no.