Inspect invalid tag

[antonioaltamura]

Hi, I read the docs about onInvalidTag, how can I access that $tag value?
Also, there is a way I can see what's the problem with that tag, some kind of error message?
This is the code, I cannot add a second 'free' tag (by 'free' I mean not chosen from the dropdown menu).
There is something I'm missing?

 <tags-input ng-model="model.authors" class="form-control"
                            display-property="lname"
                            key-property="id"
                            onInvalidTag={alert('1')}>
<auto-complete source="loadTags($query)"></auto-complete>
</tags-input>

Edit: it looks like key-property="id" is a problem in case of multiple tags not coming from the autocomplete resource. There is a way to handle this problem?