cannot handle default arguments for lamdba

this code does't work.

const buble = require("buble");

const source = `
  (function(x, f = () => x) {
      var x;
      var y = x;
      x = 2;
      return [x, y, f()];
    })(1)
`;

var result = buble.transform( source );
console.log(result.code);
eval(result.code);