Demo of a bug with nyc when using https://gist.github.com/ericelliott/f3c2a53a1d4100539f71.

package.json

{
  "name": "nyc-ava-test",
  "version": "1.0.0",
  "description": "",
  "main": "say.js",
  "scripts": {
    "test": "npm run test:node-tap && npm run test:ava && npm run test:node-tap:coverage && npm run test:ava:coverage",
    "test:node-tap": "tap test-node-tap.js",
    "test:ava": "ava test-ava.js",
    "test:node-tap:coverage": "tap test-node-tap.js --coverage-report=text",
    "test:ava:coverage": "nyc --require babel-register npm run test:ava"
  },
  "author": "",
  "license": "ISC",
  "babel": {
    "presets": ["es2015"]
  },
  "ava": {
    "require": [
      "babel-register"
    ]
  },
  "devDependencies": {
    "ava": "^0.15.1",
    "babel": "^6.5.2",
    "babel-cli": "^6.9.0",
    "babel-preset-es2015": "^6.9.0",
    "babel-register": "^6.9.0"
  }
}

say.js

module.exports = function say({
  greeting = 'Hello',
  noun = 'world'
} = {}) {
  return greeting + ' ' + noun;
}

test-ava.js

import test from 'ava';
import say from './say.js';

test('should default to hello world', t => {
  t.is(say(), 'Hello world');
});

test('should say a custom greeting to hello world', t => {
  t.is(say({
    greeting: 'Hi',
    noun: 'Phil'
  }), 'Hi Phil');
});

test-node-tap.js

var tap = require('tap')
var say = require('./say.js');

tap.equal(say(), 'Hello world');
tap.equal(say({
  greeting: 'Hi',
  noun: 'Phil'
}), 'Hi Phil');