If you call routerReducer() with no arguments to get back the initial state, it crashes with the following error:

reducer.js: Uncaught TypeError: Cannot read property 'type' of undefined

The generated code looks like this:

function routerReducer() {
  var state = arguments.length <= 0 || arguments[0] === undefined ? initialState : arguments[0];
  var _ref = arguments[1];
  var type = _ref.type;       // <==== _ref is undefined here and _ref.type causes the error
  var payload = _ref.payload;

  if (type === LOCATION_CHANGE) {
    return _extends({}, state, { locationBeforeTransitions: payload });
  }

  return state;
}

This can be fixed in the original code by adding a default parameter for the action parameter:

export function routerReducer(state = initialState, { type, payload } = {}) {

It made me wonder though what the correct behavior of Babel should be here. Should it provide safe destructuring or still leave that up to your code? I checked Traceur though and it behaves the same way, so perhaps that's not part of the spec, but it seems dangerous.