Why isn't tree shaking removing unused code from my ES6 module bundle in Webpack?

// src/math.js
        export const add = (a, b) => a + b;
        export const subtract = (a, b) => a - b;

        // src/index.js
        import { add } from './math';

        console.log(add(2, 3));

        // webpack.config.js
        module.exports = {
            mode: 'production',
            entry: './src/index.js',
            output: {
                filename: 'bundle.js',
                path: __dirname + '/dist',
            },
            optimization: {
                usedExports: true,
            },
        };