Cannot read property 'toggle' of undefined

[ebatgerel]

Description

I have a trouble with using component.

Expected outcome

Collapse content should be expanded

Actual outcome

When i click the button its throws a error.

Live Demo

Here is my html template:

 <template is="dom-repeat" items="[[items]]">
         <div class="collapse-container">
             <button on-click="toggle">[[item.content.title]]</button>
             <iron-collapse id="collapse">
                   <div class="collapse-item">
                         [[item.answer]]
                   </div>
              </iron-collapse>
         </div>
 </template>

Browsers Affected

• [x] Chrome
• [x] Firefox
• [x] Safari 9
• [x] Safari 8
• [x] Safari 7
• [x] Edge
• [x] IE 11
• [x] IE 10