underline won't trigger

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  inputValue: '',
  
  mouseUpCount: 0,
  
  mouseOverCount: 0,
  
  options: [
    'alice',
    'bob',
    'cindy',
    'david'
  ],
  
  actions: {
    onMouseOver(term) {
			this.incrementProperty('mouseOverCount');
    },
    
    onMouseUp(term) {
			this.incrementProperty('mouseUpCount');
    }
  }
});

helpers.underline.js

import Ember from 'ember';

export function underline([text, term]) {
	const textLower = (text || '').trim().toLowerCase();
  const termLower = (term || '').trim().toLowerCase();
  
  let i = textLower.indexOf(termLower);
  if (i === -1) {
    return text;
  } else {
    let prefix = text.substring(0, i);
    let suffix = text.substring(i + term.length);
    let match = text.substring(i, i + term.length);

    return Ember.String.htmlSafe(`${prefix}<b>${match}</b>${suffix}`);
  }
}

export default Ember.Helper.helper(underline);

templates.application.hbs

mouse over count: {{mouseOverCount}}
<br>
mouse up count: {{mouseUpCount}}
<br>
{{input value=inputValue}}
<br>

<ul>
{{#each options as |option|}}
	<li onmouseover={{action 'onMouseOver'}}>
    <span onmouseup={{action 'onMouseUp' option}}> 
       {{underline option inputValue}}
    </span>
  </li>
{{/each}}
</ul>

twiddle.json

{
  "version": "0.13.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.16.2",
    "ember-template-compiler": "2.16.2",
    "ember-testing": "2.16.2"
  },
  "addons": {
    "ember-data": "2.16.3"
  }
}