lcov report is not getting generated

I am running my ember tests with embercliblanket, but my lcov reports are not getting generated

pfb my blanket-option.js file

var options = {
  modulePrefix: 'my-ember',
  filter: '//.*my-ember/.*/',
  antifilter: '//.*(tests|template|config).*/',
  loaderExclusions: [],
  enableCoverage: true,
  cliOptions: {
    jsonOptions: {
      outputFile: 'test-output.json'
    },
    lcovOptions: {
      outputFile: 'tmp/lcov.dat'
    },
    reporters: ['lcov'],
    autostart: true
  }
};
if (typeof exports === 'undefined') {
  blanket.options(options);
} else {
  module.exports = options;
}

pfb my testem.js

module.exports = {
  "framework": "qunit",
  "test_page": "tests/index.html?hidepassed",
  "disable_watching": true,
  "launch_in_ci": [
    "PhantomJS"
  ],
  "launch_in_dev": [
    "PhantomJS",
    "Chrome"
  ]
};

I am running tests in cmd as ember test --test-page='tests/index.html?coverage=true'

Tests are success, but the expected lcov.dat files are not getting generated. Cant see any errors as well while running in the browser.

Any clues ?