nyc: I can't get sourcemaps to work with nyc, babel and mocha

Here is an example of what I am trying to do https://github.com/pghalliday/nyc-babel-mocha-sourcemaps

If you checkout the project and run

npm install
npm test

Then you get the following output

17:53 $ npm test

> nyc-babel-mocha-sourcemaps@1.0.0 test /Users/pghalliday/projects/github/pghalliday/nyc-babel-mocha-sourcemaps
> nyc mocha



  test
    1) should pass


  0 passing (15ms)
  1 failing

  1) test should pass:
     Error: what line number?
      at exports.default (src/index.js:1:1185)
      at Context.<anonymous> (test/src/index.js:8:5)



ERROR: Coverage for lines (50%) does not meet global threshold (100%)
ERROR: Coverage for statements (50%) does not meet global threshold (100%)
----------|----------|----------|----------|----------|----------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines |Uncovered Lines |
----------|----------|----------|----------|----------|----------------|
All files |       50 |      100 |      100 |       50 |                |
 index.js |       50 |      100 |      100 |       50 |              3 |
----------|----------|----------|----------|----------|----------------|
npm ERR! Test failed.  See above for more details.

Note that it reports the error as being thrown from src/index.js:1:1185 even though the source file clearly throws on line 2

export default function() {
  throw new Error('what line number?');
  return 'hello world';
};

I have installed source-map-support and set nyc to have produce-source-map and babel to have "sourceMaps": "inline" and mocha to --require source-map-support/register (i tried moving that require to nyc too)

Am I missing something or does this just not work. Thanks in advance