istambul nyc fails to detect any files when running ava test suite

Question

Started trying to integrate nyc/istambul into my ava test suites:

./node_modules/.bin/nyc --include modules/todo.js --es-modules ./node_modules/.bin/ava
  1 tests passed
  ----------|---------|----------|---------|---------|-------------------
  File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
  ----------|---------|----------|---------|---------|-------------------
  All files |       0 |        0 |       0 |       0 |                   
  ----------|---------|----------|---------|---------|-------------------

The output fails to list any of the files!

If I add the --all flag I get the following even though the tests cover both functions:

ERROR: Coverage for lines (0%) does not meet global threshold (90%)
----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |       0 |        0 |       0 |       0 |                   
 todo.js  |       0 |        0 |       0 |       0 | 6-30              
----------|---------|----------|---------|---------|-------------------

So it looks as if the output from the ava testing tool is not being picked up by the nyc coverage tool.

The simple setup is as follows:

/* todo.js (uses ES6 modules) */

'use strict'

let data = []

function clear() {
    data = []
}

function add(item, qty = 1) {
    qty = Number(qty)
    if(isNaN(qty)) throw new Error('qty must be a number')
    data.push({item: item, qty: qty})
  return true
}

export { add, clear }

/* todo.spec.js */
import { add, clear } from './modules/todo.js'
import test from 'ava'

test('add a single item', test => {
  clear()
  const ok = add('bread')
  test.truthy(ok)
})

I tried creating a .nycrc.json file but this didn't help:

{
  "check-coverage": true,
  "include": [
    "modules/todo.js"
  ],
  "reporter": ["text", "html"]
}