ES6 export and import name functions?

I have some functions and I want to keep them in an external js file. eg.

in functions.js

var double = function(x) {
  return x + x;
}

export { double };

Then in my main js file:

import double from './functions';

...

double(2)

I get this error:

Uncaught TypeError: (0 , c.default) is not a function
    at bundle.min.js:44

When I read the line 44:

(0, _functions2.default)(2);

Any ideas why? What have I missed?