Angular multiple checkboxes not passing to model

Within my application I'm allowing users to select a checkbox and also choose additional items from a further list for submission. I'm using the checklistModel directive found at http://ift.tt/1lmEPlk

My current form set up is as follows

<fieldset>
    <legend class="mb">Please email me the following brochure:</legend>
    <ul>
        <li>
            <input type="checkbox" checklist-model="formData.selectedFiles.files" checklist-value="selectedFile.id" >
            <label>{{ selectedFile.id }} {{ selectedFile.title }}</label>
        </li>
    </ul>
</fieldset>
<fieldset>
    <legend class="mb">I would also like information on the following products:</legend>
    <div class="layout">
        <div class="layout__item one-half" ng-repeat="files in alternativeFiles">
         <ul>
             <li ng-repeat="file in files">
                <input type="checkbox" checklist-model="formData.selectedFiles.files" checklist-value="file.id">
                <label>{{ file.title }}</label>
            </li>
         </ul>

        </div>
    </div>
</fieldset>
{{ formData }}

The main section within the ng-repeat for the checkbox list populates the formData fine with an array within {{formData}} populated with correct values. However, if the initial please email the following brochure checkbox is select it passes through a null value.

I have checked and confirmed that the selectedFile.id value for this item is populating correct and if I simply change the value simply to a string or fixed numeric value then the array populates.

Been looking at this for a while and have been tearing my hair out, any suggestions would be very much appreciated.

Thanks in advance.

Simon