Karma-Coverage and Istanbul HTML report not output to expected directory

I am having problems getting Istanbul to output it's HTML report to the directory specified in my Karma config. It currently outputs the main index.html in the correct directory(reports/coverage) but the detailed html child pages are output to the same directory as the source file(www/js/...). It is the same problem described here, but after trying the suggested solution the problem persists.

This is the contents of my karma config file:

basePath: '',

frameworks: ['jasmine'],

files: [
  '../www/lib/angular/angular.js',
  '../www/js/*.js',
  '../www/js/services/*.js',
  '../www/js/controllers/*.js',
  '../www/lib/angular-mocks/angular-mocks.js',
  '**/*test.js',
  'test-data/*.json'
],

plugins: [
  'karma-jasmine',
  'karma-coverage',
  'karma-phantomjs-launcher',
  'karma-ng-json2js-preprocessor'
],

preprocessors: {
  '../www/js/**/*.js': ['coverage'],
  '**/*.json':         ['ng-json2js']
},

ngJson2JsPreprocessor: {
  stripPrefix: 'test-data/',
  prependPrefix: 'served/'
},

reporters: ['progress', 'coverage'],

port: 9876,
colors: true,
logLevel: config.LOG_INFO,
autoWatch: true,
browsers: ['PhantomJS'],
singleRun: true,

coverageReporter: {
  // Common output directory
  dir: 'reports/coverage',
  reporters: [
    { type: 'text-summary'},
    { type: 'html', subdir:'report-html' }
  ]
}

Suggestions on how to resolve this so all of the coverage files are placed into the reports/coveragedirectory would be much appreciated. Thanks.